Well the 6800GT by far, it will also easily OC to ultra speeds.
Also if you want to play levels like HL2 HDR or any shader model 3 based games, you need an nvidia card to use these graphics features. Such games are farcry and splinter cell chaos theroy, and HL2 is HDR (Soon)
Your looking at a very expensive overepriced GT as well
EDIT: Just search for 6800GT on newegg.com, you can pick up the best 6800GT which is an eVGA which is 300$ after a 30$ rebate