Killer gaming machine listed in 10 steps.
1. Graphics Card-The 6800GT or Ultra is apparently the way to go right now.
2. Motherboard- Make sure to get a good motherboard that has the things you may think you will need like gigabit lan and usb ports,firewire or whatever. the 6 channel or more audio generally sounds pretty good. Um...don't get a crap board either. You'll get what you pay for. get an ASUS or Soyo or something like that.
3. Processor-I think AMDs do much better than intels. I've heard mixed things about 64bit gaming. I've heard it's a marketing gimmick and i've heard it helps. Mostly from what I can tell though 64's just hold a lot more ram and aren't really faster. I hope someone can clear that up.
4. RAM- It's good to get good quality RAM. I would get corsair xms or kingston hyper-x. There is another brand made specifically for AMDs that I just can't think of at the moemnt that is suppose to be good, but yeah I would go with AT LEAST a Gig of Ram made for gaming like Kingston Hyper-x or Corsair's XMS. I can tell a small difference in "gaming" ram and the other but it's a nice difference to me.
5. HDD-a Serial ATA drive is probably optimal at 10,000rpm. Of course these are quite expensive. 7200rpm SATA drives are much less expensive and the margin of performance loss is minimal.
6. Killer Cooling-The icing on the cake of a killer gaming system is watercooling. Seriously OCing!! If you don't do that make sure you get good fans.
7.CD Drives-Probably want to get a DVD(rw if you want) drive for quick installation of the latest games that take up a lot of real estate on the HDD like UT2004. Also a lot of demos for games are coming out on DVD and it's nice to be able to try them out. I would probably go ahead and get a cd-rw drive for quick disc copying.
8. Case-Get an awesome looking case with a clear side case to see all your cool parts. Make sure it has a good size power supply(at least 400w).
9. Sound Cards-While Mobo integrated audio doesn't sound bad, a good sound card will really make gaming awesome.Audigy 2 Platinum or something is awesome. You can get just a good quality card for cheaper though IMO.
10. Accessories-Get some cool looking stuff like Round IDE cables for when you have your clear side on your case. I think they really set a machine's inside looks off. Get some fan covers that you dig. There is a lot of things to make your comp look cool asthetically and yeah if nothing else it gets attention for how friggin' cool it looks. Don't show it to girls though because sometimes it um...makes them thing you are a nerd...i know mine does.