There has been the expected torrent of threads asking whether upgrading to an 8800GT requires a faster CPU. Well here's an article that can provide much of the answer. I learned a long time ago that the CPU is irrelevant at the video settings I game at and this seems to be the case still. If you like to work out your video card you can get by with a good graphics card and a cheapo dual-core.
http://www.firingsquad.com/hardware/$500_gaming_pc_upgrade/page5.asp
http://www.firingsquad.com/hardware/$500_gaming_pc_upgrade/page5.asp