ASN, good points but you also forgot to add:
SLI video cards (where two cards run in tandem)
Fast harddrive ......look at the rpm, seek and retrieve times and cache...
Fast and powerful sound card such as the Soundblaster Fatality X-FI. I installed this one and all my games run about 25% faster now. (plus the remote control is cool when you're listening to tunes).
The ram must be DD2 and all the specs should be close to 2,2,2.5,2 (or lower)
If I was building a gamer, I'd go to somewhere like Canada Computer and tell them what you're after and have them build it from all the best components.
I'd also check out gamer sites like gamespy and the like for their ultimate gaming machines.
Don't forget you'll probably want to overclock it too so water cooling is something to be considered for the processor, the northbridge, the hard drives and the memory.
If you're going to be playing online you'll need a 5 mbps connection or better
I'd also look into the specs on AMD based machines. AMD is supposed to be THE architecture for gaming but someone told me recently that an intel based machine matched their performance at less cost.