I'm trying to get this topic stickied, so if you see anything that can be improved/changed let me know.
I'm getting a little tired of seeing the countless threads about gaming builds, if their any good or not. Sure I made a few of my own, but now I'm much more knowlagable and would like to pass that off to the "newbs."
Since this is a gaming computer, AMD and SLI is where it's at.
DFI Lanparty nF4 SLI-DR
of course you can get away with the non-SLI version if you don't want dual video cards.
DFI Lanparty UT nF4 Ultra-D
This is your best bet for buying a motherboard. It's easily overclockable, which is a plus for most gamers.
Asus A8N-SLI Premium
GIGABYTE GA-K8N Pro-SLI
You've got basicly three options here, in my opinion.
AMD Athlon 64 3700+ San Diego
AMD Athlon 64 X2 4400+ Toledo
AMD Athlon 64 X2 3800+
I decided on the 3700+ because if you don't mind overclocking a bit, you can get up to 2.4GHz. and save ~$90 by not getting the 4000+. The two dual core X2 chips are more futureproof, so when games start supporting Dual Core CPUs, you'll be ready.
Okay, first thing you want to consider is how much money do you want to spend on your video card? You've got a couple of options. These can all be used in SLI mode, to achieve an even greater gaming PC than a single card PC.
eVGA GeForce 6600GT
eVGA GeForce 6800GT
eVGA GeForce 7800GT
eVGA GeForce 7800GTX
Any of these cards would do, although I wouldn't choose anything less than a 6800Ultra if you want to play upcoming games with all the settings on High.
You'll want some DDR 400 (PC 3200) Ram. 2GB's should be plenty. OCZ and Mushkin are good brands.
You'll want a nice big hard drive. SATA-150 is recommended. A 200GB drive should do. Also, some people suggest a 74GB 10k drive for the OS and games. This isn't required, but does speed up your PC.
That's basically the "Ultimate" gaming machine. You'll need a case, PSU, Optical Drives, and some speakers and a moniter also. These things are not the major factor in a gaming PC tho, as are the items listed above.