I have no performance gain expected. Crysis, the game oar systematically from the moment I turn on AA or I put myself in 1920x1080. (I have a screen 23 ").
I tested Crysis because I thought the performance limitations on the demo Crysis2.
A friend who has an ati 4870 have better performance (fps does not drop below 30) than me who have an average of 42 FPS with falls was 20 ...
Here is my config:
Core2Duo E8400 3.0ghz
Gainward gtx570 phantom
4GB dual channel kingston (1080mhz)