I'd suggest anything Intel over AMD. Intel's current chips just simply outperform the AMD offerings. Hopefully this future release of Phenom II (AMD) chips will change things a bit.
Until then, I'd suggest an Intel chip. If you go dual-core, go with a E8400 or better. For quads, the Q6600 is a good chip for a decent price.
You'll probably want to try to overclock your CPU to 3Ghz+ if you can, to get full potential. Even with my 8800GTS I showed FPS improvement in Crysis by going from 2.4Ghz (stock Q6600) to 3.2Ghz.