Brtnboarder is pretty accurate. Maxed out on 1280x1024 on BF2 I get 35-45FPS. Its lower for other games. FEAR is horrible on a 6600GT when maxed out (then again, it even gives 7800GTs a run for their money). I could use a 6800GS, but the 6600GT isn't bad at all. Infact, it was the videocard of choice for 75% of systems around $1000.
The 7800GT is huge step up from the 6600GT, but it can be considered over the top. Right now, a 6800GS wouldn't have any trouble at all in running 99% of the games out there at max settings. There is always a graphics card that is over the top of whats needed. Look at Geforce 6, its almost done with 2 years of its life and its still running games like BF2 and CoD2 and Quake 4 at max settings. GeForce 7 is just a sort of future-proofing thing, and can be considered a fab. That, or if you NEED to get like 80FPS in a game...
Don't look at FEAR. It IS NOT a measure of things to come. That game was made overly demanding. Even 7800GTXs get like 30FPS on max settings on there. I'll bet you that even Unreal Tournament 2007 won't be as demanding as FEAR is. Well...maybe close
You should get 40FPS with everything maxed out on 1280x1024 on FEAR with a 7800GTX.