Well, you seem a little vague and misguided...let me explain it all.
First you should know this: a PSU is not like the next PSU, the £100 onesa are that expensive for a reason...that reason is that they work proeperly how they should. Your £30 or ones that come in a case are a pile of garbage. I can't even begin to stress how pricee=quality in PSU more than probably any other component.
Now that that is done, what you need in a PSU and what to look for. The PSU wattage is the highest possible wattage that unit can output, however not all units can sustain that output, a £130 850W PSU can easily handle full load, a £20 850W PSU can't hardly handle 50% load.
12V rails are the ones that you are interested in, the ampage on them to be more precise. If you are running a modern day gaming graphics card, 8600, 8800, 9800, 200 series etc you will ned atleast 30A on the 12V rail(s).
If you have compined 12V rails, so more than 1 12V rail, they don't add up, a PSU with 2 30A 12V rails doesn't mean it has 60A, it means each rail can handle 30A at most.
That last thing is important because when one peaks and tries to go higher it can't and the PSU shuts down, as does your system. If you have 1 dedicated 12V rail at 70A then it can handle up to 70A of hardware before it cuts out....that is alot in a computer.
As for compatability, it depends on your motherboard and video cards:
The motherboard has 2 connectors for the power supply, either a 20 or 24 pin connector and either a 4 or 8 pin connector. Most power supplies now come with a 20+4 and 4+4 pin connector. What this means is that it can be used on any motherboard because it has 20 pins and then an aditional 4 pins which can be clipped on to give you 20 pins+4 more, 24 pins altogether. Same idea for 4+4, it is the same as having a 4 and 8 pin connector that you can change.
For the video cards, some more modern video cards have ther own power connectors. If this is the case you will need to make sure that your power supply has these connectors. They come in 6 and 8 pin varrieties and again alot of power supplies now give 6+2 pin connectors for greater compatability. The video card cables are called PCI-E pins.
If you keep all of that in mind, you should be able to gete a solid power supply that is compatible with your rig, and remember that first thing i said, PRICE=QUALITY. Don't go and get a cheap PSU that is 750W and think it will act like a 750W PSU, it won't, it will at best be like a 500W, the more expensive 750W PSUs are capable of running at 750W under load and still provide clean power and not overheat
