Lol, the cpu won't be letting the comp down man. A good AMD Athlon64 3200+ venice core cpu, or even a 3500+ if you have some spare cash is great. I havnt' seen amy benchmarks with dual core vs single core, so I dunno how much better these dual core cpu's do in gaming anyways. Lol, nah, for gaming stick with a Amd Athlon64 dude, saw some benchmarks, the single core FX57 beat the dual core Athlon64 x2's pretty much every time by a good framerate too. So yeah, dual core expensive, and not really built for gaming.. well, at least not yet.
since you have a big budget (I'm guessing USD) then get like a nvidia 6600GT, I'd say sli so there's room for upgrading there if the need ever arises.
a good dfi sli socket 939 mobo would be great.