Praetor

Power Supply 101

Revision History
1.00 - March 2005
1.10 - July 2006

Contents

1. What do all those fancy words mean?
2. How do I go about picking a nice PSU?
3. I'm lazy! Which PSUs do you reccomend?
4. Official Stuff
5. VFAQ
6. Addendum

Thanks to
ceewi1
mgoldb2
hairy lee

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 01 - What Do All Those Fancy Words Mean?
PSU
Power supply unit. This is the thing that you plug into the electrical outlet and provides your computer with power.

PFC, Active PFC, Passive PFC
PFC is an acronym for power factor correction Before I can explain what it is, we need to have a little background: your computer is connected to an electrical outlet which provided power via an alternating current (AC). Now before the computer can use this power, the waveforms needs to somehow be synchronized. Simple enough. Now what happens when the power waveforms are out of sync? The power supply has to somehow adjust the signals so that the it can provide (relatively) clean and stable power to the rest of the computer.

• In a power supply with no PFC, the synchronization is done using capacitors. Using capacitors gets the job done however it's not ideal because capacitors introduce noise and voltage drops at peak (and dropping the voltage drops the total amount of power available). The laymans explaination is that there is a reduction in the effiency of the PSU
• In a power supply that has passive PFC, they add some coils that work alongside the capacitors. By having these inductors along with the capacitors, the waveform synchronization is done with less noise being generated. Power supplies with passive PFC require you to "calibrate" them by setting the input voltage (generally there is a secure toggle switch for 115/230v). Even with passive PFC there can still be noise when dealing with 115V power.
• For power supplies with active PFC use "fancy circuitry" to make a dynamic power regulator (which, essentially means you dont need to set the input voltage) and generally has a higher efficiency rating than power supplies with only passive PFC.

From a technical perspective, of the available types of PFC, active PFC results in the most efficient power supply. Of course, whether a PSU with PFC is even needed is up for debate (and will be dealt with later in the VFAQ)

Rails
Power supplies dont just deliver one voltage, they deliver a bunch. Each one of those output voltages is a rail. The most common are +3.3v, +5v and +12v. Of these three rails, the most critical (and often overlooked and forgotten) is the 12v rail.

Twin Rails, Dual +12v, etc
Every power supply has multiple rails however, since we are only seriously concerned with the 12v rail (and thus, colloquially, when we refer to "rail" it almost always means the 12v one) it is nice to have more than one such 12v rail. Why? Because the voltage fluctations on each rail are independent from the next. This means that if you really stress one rail, say with a high end videocard, the power output on another rail will not suffer voltage stability problems.

Many good power supplies will feature dual 12v rails (having more than two is generally an indicator of a very heavy duty power supply) however it should be noted that having two rails does not guarante the power supply to be good nor does the lack of dual rails mark that power supply as being bad. The issue of whether or not dual rails are needed/benificial will be tacked in more detail in the VFAQ

Adjustable Rails/Pots
Although the power supply itself will attempt to deliver power that as close to the 3.3, 5 and 12 spec as possible, high end power supplies will come with adjustable pots (short for potentiometer) which allow users to manually tweak the output voltages so that the power supply in fact, actually output an exact value.

Hold-up Time
In an idea world, PSUs deliver nice clean power, 24x7x365 but in realiy that may not be the case and for very brief moments, there will be a flicker in power. The hold-up time rating of the computer indicates the duration that it can handle without power. Generally speaking 16s is standard with some exceptional power supplies being able to "pretend nothing happened" for up to 20ms and 24ms.

Peak Rating
The peak rating of a PSU is just that, the amount of power that a PSU can deliver for a very brief amount of time (usually less than a minute). Cheaper lower quality power supplies will tend to advertise peak ratings in place of their sustainable ratings.

ATX12, ATX20, ATX24 etc
These terms are generally in reference to the physical cables coming out of the power supply itself. The main cable which provides the bulk of the power to the PSU is most generically rferred to as the ATX main and sometimes specifically referred to as ATX20, ATX24 or ATX20+4, the extra numbers which indicate the number of pins on the connector.
When Intel released the Pentium4, the processor required additional power and a new power cabled was needed to provide this power (and dubbed the "P4 power connector"), this is a 4-pin connector. Note that although this may commonly be referred to as the P4 power connector, that all modern systems require this connector.
With the advent of high powered, PCI Express based videocards, it was found that even the PCI Express bus was not able to deliver enough power to the system

Tolerance
The tolerance of a power supply refers to how tightly regulated the voltage rails are: in short, it is a measure of how clean the power is. The rating is generally given as a percentage with the lower the value, the better. The ATX specification requires that the voltage be delivered within ±5% of the stated value (and ±10% for the 12V line under load but that's more of a technicality). Most decent power supplies will supply voltages within an even tighter ±3% tolerance and some even go so far as to provide within a ±1% tolerance.

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 02 - How do I go about picking a nice PSU?
What's Important? Whats more important than what?

1. Current distribution. Look at the power supplie's sticker, label or website. If it's the sticker you're looking at, you'll see something similar to this:
   
   
   • What if there is no sticker? What if the website doesnt say? Then the power supply is, generally speaking, crap. Don't buy it.
   • Ok I can see the ratings? How do I know if it's good? Looking at the ratings you'll see an entry for "+12v" or even multiple entries (i.e., 12V1, 12V2, 12V3 etc). Although there are some exceptions for the cases where there are multiple entries, just for a general feeling of the power supply, simply add up all the currents (measured in amperes, and usuall abbreviated with a A). In the example above, there are two 12V entries each with 18A so we have a sumtotal of 36A available to the 12V rail.
     
     As an absolute minimum, make sure the rating on the 12V rail is at least 18A. Dont' bother even considering te power supply if it's running less than this. Now note that this absolute minimum applies to people building extremely low-budget systems and for the rest of the buyers, strive to ensure that the rating is at least 22A (and given the pricing of 22A-28A PSUs there's not much reason to settle for an 18A one). Naturally, the higher this rating the better.
2. Voltage tolerance and hold-up time. Generally a bit more difficult to find out (especially the voltage tolerance), strive to get a power supply with a lower tolerance and a higher hold-up time. As noted before, the tolerance needs to be within a ±5% spread and the hold-up time should be at least 16ms.
3. Brand and reputation. Let's face it, not all power supplies makers produce high quality products and by the same token, not all the products made by a company are made equally. As such I am somewhat relucatant to have a listing of "good" brands. You can get a feel for some good brands by looking through the reccomendations
4. Wattage. So many people say "get a 500W PSU" or whatever but fail to realize that the wattage means nothing if all that power is being provided on the wrong rail. A case in point, consider the Thermaltake PurePower 480W - a decent wellknown brand, a seemingly decent wattage rating but looking a bit deeper we find some flaws:
   • Uses the older 20pin main connector rather than the 24pin or 20+4 connector
   • For all that wattage, this power suppy has a measily 18A on the 12V rail
     With the rest of the features being non-exceptional. So all in all? Not the greatst buy. But could we have done better? Absolutely. Looking at the cheaper Antec TruePower TPII-480 we have a cumulative 36A on the 12V line, better efficiency. better hold up and almost certainly better voltage regulation.

While not so much a point to look for but as an overal thing, make sure any power supply you buy uses either a 24pin (native) connector or a 20+4pin connector. This will ensure that your power supply will be somewhat useful as newer and newer computers come out. So, a summary:

What Makes a Good PSU?

• Never ever, under any circumstances, seetle for less than 18A on the 12V line, strive to maintain a minimum of 22A on the 12V rail where possible, naturally the higher the better
• Look for a power supply with a solid warranty: shoot for at least 3 years -- it's an indication of how much the manufacturer will stand behind their product
• Stuff like PFC and Active PFC, while nice should be treated as bonuses: a power supply without such circuitry will still be able to provide clean, reliable power to your equipment
• Make sure the holdtime is at least 16ms, some manufacturers sneak in seemingly good values everywhere else and skimp on the hold-time
• Tight voltage regulation: if you're buying a decent power supply to start with you should be able to count on the bare minimum of ±5% tolerance however if you've come across a power supply with a tighter advertised rating, all the better! As for power supplies with adjustable pots, you can generally count on them having a voltage regulation of at least ±3%

Special Considerations

• Unless you know what you're doing, try to avoid fanless/passive-cooled PSUs. The efficiency of a power supply as well as the stability of the rails is dependent on the operating temperature of the psu (i.e., the hotter it gets, the less power efficiency you'll have). Unless you absolutely need dead silence, go for a reduced-noise ouput PSU with a larger fan (the larger fan can provide the same amount of airflow for reduced rpms and thus reduced noise)
• For those of you looking for power supplies with modular cables in order to tidy of the mess: modular power supplies are often not as good as the non-modular variety. The reasoning is that, by making things modular, you increase both the resistance at each of the joints (meaning a power loss) as well as the number of potential failure points in the power supply.

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 03 - I'm lazy! Which PSUs do you recommend?
Budget
PSUs in this category must meet the following criteria

• Cost less than $50
• Must provide at least 18A on the 12V rail
• Must provide the minimum 16ms hold-up time
• Must provide a net of 400W
• Must be either 24p or 20+4p

Since these are going to be the bottom of the barrel PSUs (in terms of quality), I'll probably stay away from modular and/or "silent" PSUs -- the goal is to find a quality PSU for as little money as possible ... if you're looking for a bling-bling PSU on a tiny budget, this guide isn't for you.

HIPRO HP-430PA3 True 400W
Cost: $19.99
Rails: +3.3V@27A, +5V@22A, +12V@[14+13]A
Mobo: 24p
Cooling: 1x80mm
Comments: Dirt cheap, some pretty impressive on-paper specs, this is a hell of a PSU for it's price. Although I originally noted that the minimum spec for PSUs would be 18A on the +12V line, this PSU really sets the bar high especially for the price.

Logisys PS550ABK 550W
Cost: $19.99
Rails: +3.3V@30A, +5V@38A, +12V@25A
Mobo: 24p
Cooling: 2x80mm
Comments: Nothing too brilliant about this PSU other than it's price tag. I included it as a price-point alternative in case the HiPRO goes out of stock.

HIPRO HP-E4009F5WR ATX V2.0 True 400W
Cost: $28.99
Rails: +3.3V@30A, +5V@28A, +12V@[14+15]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: A solid enough PSU with an appealing pricetag.

Rosewill RV450-2 450W
Cost: $31.99
Rails: +3.3V@30A, +5V@45A, +12V@[17+16]A
Mobo: 20+4p
Cooling: 2x80mm
Comments: A fairly solid PSU all around.

HIPRO HP-P500W/TOP-500P5 500W
Cost: $36.50
Rails: +3.3V@28A, +5V@44A, +12V@[20+20]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: A fairly solid PSU, with a total +12V@29A.

XClio 450BL 450W *** Praetor's Pick ***
Cost: $39.99
Rails: +3.3V@32A, +5V@30A, +12V@[15+17]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: One of my most popular recommended PSUs this PSU has it all for the price bracket (tight voltage regulation, 20ms hold-up, high power efficiency, solid rails). This PSU is highly recommended.

Coolmax CR-450W 450W
Cost: $42.99
Rails: +3.3V@30A, +5V@45A, +12V@[17+16]A
Mobo: 20+4p
Cooling: 1x140
Comments:With the larger fan, this is a good low-budget noise-optimized PSU that doesn't sacrifice performance

Midrange
Since I was pleasantly surprised by the some of the decent PSUs I rounded up in the previous category, the bar has definitively been raised for this bracket. PSUs recommended here must

• Cost less than $75
• Provide 28A or better on the +12V line
• Aim for ±3% voltage tolerance
• Must either be 24p or 20+4p
• Provide a minimum of 500W net load

Athena Power AP-P4ATX50F12 EPS12V 500W
Cost: $49.50
Rails: +3.3V@35A, +5V@61A, +12V@28A
Mobo: 20+4p
Cooling: 1x120mm+1x80mm
Comments: A decent PSU all around, the PSU gets both exceptional cooling as well as not generating significant noise.

Coolmax CP-500T EPS12V 500W
Cost: $49.99
Rails: +3.3V@30, +5V@50A, +12V[16+18]A
Mobo: 24p
Cooling: 1x120mm
Comments: Another solid PSU, this one qualifies for EPS12V systems and also features Active PFC.

Atrix PSAX-550BL 550W
Cost: $49.99
Rails: +3.3V@32A, +5V@40, +12V@[19+20]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: A very robust PSU with some very impressive stats however the MTBF stands out: the 25ºC ambient temperature that it (the MTBF) is measured at is significantly lower than the operating temperature that PSUs usually run at

XClio GOODPOWER 500W
Cost: $51.99
Rails: +3.3V@30A, +5V@28A, +12V@[16+17]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: For those that crave it, this PSU has SLI Certification but more importantly, is a solid PSU.

Rosewill RP550-2 500W
Cost: $54.99
Rails: +3.3V@30, +5V@50A, +12V[18+18]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: This is a bling bling PSU without sacrificing both quality or noise. No complaints here, a solid recommendation all around.

Rosewill RP500-2 500W
Cost: $58.99
Rails: +3.3V@30, +5V@45A, +12V[18+16]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: This is a bling bling PSU without sacrificing both quality or noise. No complaints here, a solid recommendation all around.

Rosewill RT550-135-BK 550W
Cost: $59.99
Rails: +3.3V@30A, +5V@25A, +12V@[17+18]A
Mobo: 20+4p
Cooling: 1x135mm
Comments: EPS12V certified and SLI ready. A robust, quality PSU.

Sunbeam NUUO SUNNU550-US-SV550W *** Praetor's Pick ***
Cost: $59.99
Rails: +3.3V@28A, +5V@36A, +12V@[20+18]A
Mobo: 20+4p
Cooling: 1x80mm+1x120mm
Comments: Tight ±3% voltage regulation, Active PFC, SLI Certified, PSU is very solid all around and if the 3.3v line was slightly more robust, could be a contender for the next category.

Enhance ENP-5150GH 500W
Cost: $69.01
Rails: +3.3V@25A, +5V@20A, +12V@[18+18]A
Mobo: 20+4p
Cooling: 1x120mm
Comments: 80+ certified, EPS12V certified, Active PFC and SLI ready. Without a doubt one of the best PSUs in this catagory.

XCLIO STABLEPOWER 500W
Cost: $69.99
Rails: +3.3V@30A, +5V@30A, +12V@[18+18]A
Mobo: 20+4p
Cooling: 1x140mm
Comments: Although, IMO, not quite as good as the Enhance unit directly above, this is easily available from newegg and remains an excellent quality unit.

Enthusiast
PSUs in this bracket don't have a price limitation but they do have some stringent requirements

• Must have an exceptional and well known record of solid performance. This is by far the most important requirement.
• Must have a minimum of 36A on the 12V line spread over at least two rails
• Must provide an excess of 500W net
• Must have two PCI-Express connectors
• High preference for adjustable rails
• HIgh preference for ±3% voltage regulation
• High preference given for longer warranties
• Must have some form of PFC either active or passive

In short, this list will be incredibly picky and every speci will be taken into consideration.

Antec NeoHE 500W
Cost: $99.99
Rails: +3.3V@23A, +5V@17A, +12V@[17+17+17]A
Voltage Regulation: ±3% marketed, ±1% common
MTBF: 80,000+ @ 50ºC
Hold-up: 20ms
PFC: Active
Mobo: 20+4p
Warranty: 3 years
Cooling: 1x80mm
Features: SLI Certified, Modular
Comments: The NeoHE series of boards has a known issue with some last generation ASUS boards (which, I believe have been addressed by ASUS/Antec)

Antec NeoPower NeoHE 550W
Cost: $109.99
Rails: +3.3V@24A, +5V@20A, +12V@[18+18+18]A
Voltage Regulation: ±3% marketed, ±1% common
MTBF: 80,000+ @ 50ºC
Hold-up: 20ms
PFC: Active
Mobo: 20+4p
Warranty: 3 years
Cooling: 1x80mm
Features: SLI Certified, Modular
Comments: As same with the 500W model of this series, there are known issues (which I believe have been addressed) with using the NeoHe series of PSUs with certain high end ASUS motherboards

OCz GameXStream 600W
Cost: $114.99 (after $25.00 MIR)
Rails: +3.3V@36A, +5V@30A, +12V@[18+18+18+18]A
Voltage Regulation: ±5% marketed, ±1% effective
MTBF: 100,000+ @ 25ºC
Hold-up: 17ms
PFC: Active
Mobo: 24p
Warrany: 3-year
Cooling: 1x120mm
Features: Crossfire Ready
Comments: Building upon the very well esteemed PowerStream series, this lineup switches to a 24p connector, adds PFC and quad rails (the PowerStream600W had dual rails however)

OCz GameXStream 700W
Cost: $119.99 (after $35.00 MIR)
Rails: +3.3V@36A, +5V@30A, +12V@[18+18+18+18]A
Voltage Regulation: ±5% marketed, ±1% effective
MTBF: 100,000+ @ 25ºC
Hold-up: 17ms
PFC: Active
Mobo: 24p
Warrany: 3-year
Cooling: 1x120mm
Features: Crossfire Ready
Comments: With a greater total +12V output than its 600W sibling, this PSU is extremely robust and definitively one of the best in this category.

Corsair HX520W
Cost: $119.99
Rails: +3.3V@24A, +5V@24A, +12V@[18+18+18]A
Voltage Regulation: ±5% marketed, 1% common
MTBF: 100,000+ @ 50ºC
Hold-up: 16ms
PFC: Active
Mobo: 20+4p
Warranty: 5 years
Cooling: 1x120mm
Features: SLI Certified, Modular
Comments: Despite being labeled to the contrary, this unit is capable of putting as much of the total +12V@40A on any rail as is required. Rated at 50ºC it is a superb PSU.

Corsair HX620W
Cost: $154.99 (after $15 MIR)
Rails: +3.3V@24A, +5V@30A, +12V@[18+18+18]A
Voltage Regulation: ±5% marketed, 1% common
MTBF: 100,000+ @ 50ºC
Hold-up: 16ms
PFC: Active
Mobo: 20+4p
Warranty: 5 years
Cooling: 1x120mm
Features: SLI Certified, Modular
Comments: With a greater total +12V output than its 520W sibling, this is a superb PSU.

XClio GreatPower X14S4P4 750W
Cost: $174.99
Rails: +3.3V@30A, +5V@28A, +12V@[18+18+18+18]A
Voltage Regulation: ±3-4% marketed, 2% common
MTBF: 100,000+ @ 25ºC
Hold-up: 16ms
PFC: Active
Mobo: 20+4p
Warranty: 3 years
Cooling: 1x140mm
Features: SLI Certified, Modular, Adjustable Pots
Comments: Carrying on the expected high quality of some of the other XClio PSUs, this is no exception. It was a bit difficult finding some of the information however.

SILVERSTONE SST-ST75ZF 750W
Cost: $184.99
Rails: +3.3V@24A, +5V@30A, +12V@[18+18+18+18]A
Voltage Regulation: ±5% marketed, 1% common
MTBF: 100,000+ @ 25ºC
Hold-up: 17ms
PFC: Active
Mobo: 20+4pin
Warranty: 3 years
Cooling: 1x80mm
Features: Active PFC, SLI Certified, EPS12V certified
Comments: A top quality unit.

Silverstone ST85ZF 850W
Cost: $244.99
Rails: +3.3V@24A, +5V@30A, +12V@[18+18+18+18]A
Voltage Regulation: ±5% marketed, ±1% common
MTBF: > 100,000+ hours @ 25ºC
Hold-Up: 17ms
PFC: Active
Mobo: 24pin
Warranty: 3-year
Cooling: 1x80mm
Features: SLI Certified for all configurations, 4 PCI-E connectors.
Comments: A top quality unit, despite low efficiency at low loads

Silverstone OP1000 1KW
Cost: $344.99
Rails: +3.3V@30A, +5V@40A, +12V@[80]A
Voltage Regulation: ±5% marketed, ±1% common
MTBF: > 100,000+ hours @ 25ºC
Hold-Up: 16ms
PFC: Active
Mobo: 24pin
Warranty: 3-year
Cooling: 1x80mm
Features: 6 PCI-E connectors.
Comments: Rated at 50C, this is a top quality 1KW PSU without the obscene pricetag of the PC P&C.

PC Power & Cooling TurboCool 1KW-SR
Cost: $549.00
Rails: +3.3V@24A, +5V@30A, +12V@72A
Voltage Regulation: ±2% marketed, ±1% common
MTBF: 100,000+ @ 50ºC
Hold-up: 32ms
PFC: Active
Mobo: 24p
Warranty: 5-years
Cooling: 1x80mm
Features: SLI Certified
Comments: Replacing the older TurboCool 1KW, this unit features a powerful single +12V rail. This PSU thoroughly trumps most others in every single respect whether we judge by real-world tolerance, power load, MTBF, warranty and most impressively, hold-time (50% better!). The only downside is the obscene price tag.

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 04 - Official Crap
ATX 1.3 Specifcation
ATX 2.01 Specifcation
ATX 2.2 Specifcation
EPS12V 2.1 Pedestal Specifications
EPS1U Rev. 2.1 1U non-redundant
EPS2U Rev. 2.1 2U non-redundant
ERP12V Rev. 1.4 - Pedestal Redundant
ERP2U- Rev. 2.0 2U Redundant

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 05 - VFAQ
How many Watts Do I Need??
The answer is dont ask that question. That's right. Don't ask that question. Why not? Because it's almost pointless. Why? If you read the section where I described how to pick a good PSU (hint hint) then you'd know what to look for. A recap (or should I say, introduction? )

• Make sure the PSU has, at the absolute least, 18A on the 12V line. In reality, given what you can get for the money you spend, you should set this bar at 24A. If you need suggestions or inspiration, see the above suggestions
• Make sure the PSU has 16ms or better hold-up time. Just because a PSU seems to have decent numbers everywhere else doesnt mean there's no catch. This is an example of such a scenario -- the PSU has the minimum 18A on the 12V line and is SLI Certified and all but only has 12ms hold-up. Dont get it!
• With the above two points in mind as a minimum (more points in the section detailing how to pick out a good PSU), when you pick a PSU that matches those criteria you'll end up with "a healthy number of watts" so dont stress it (i.e., there is no 250W PSU or whatever with the above characteristics). So just match the 12V rail and the hold time and for the most part you'll be ok
• Now if you've read the above and you're absolutely dying still to get a number (because for some reason, you think a big number in the watts column is going to give you performance, then use 450W.

Is my PSU strong enough for my setup?
Again, see the above FAQ, this is just a rehash of the same thing. More particular however, make sure you spend an appropriate amount of money on the PSU as you do the rest of the system How much is appropriate? Common sense! For instance, if you're spending $1000 on some fancy SLI configuration and you're buying a $30PSU, something's not gonna work so well. Here is a very rough ballpark guide:
- Low end videocard ... shoot for 24A (to a absolute minimum of 18A)
- Midrange single videocard ... shoot for 28A
- Midrange dual-videocard ... shoot for 32A
- High-end setups (any) ... shoot for 34A+
It seems like stringent requirements/guidelines but once you start looking at the products you'll quickly realize it's not that unaffordable.

I have a SLI/Crossfire setup ... does this mean I need a SLI/Crossfire certified PSU?
No. Just get a PSU with a sufficient enough number of amps on the 12V line and you'll be fine! If you reall insist on and want a number, I reccomend 34A as a minimum for two GPU and 38A as a minimum for quad-GPU.

__________________
ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

Praetor

Section 06 - Addendum
Where is the Voltage Being Used?
PSUs come with a handful of rails, sometime's it's nice to track them down:
+12V -- Pretty much anything with a motor, the P4/ATX12 connector
+5V -- General motherboard stuff, memory, laptop harddrives
+3.3V -- CPU core, memory
-12V -- Used mostly older serial chips
-5V -- Obsolete/discontinued

I just built my computer and it wont turn on!
Plug in all the cables correctly, including (where applicable) the EPS12 (a 2x8 pin connector) and the ATX12 (2x2 pin connector)

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ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8
4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400
5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10
OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI