My Q6600 with an 8800GTS 512 plays crysis just fine on medium with 2xAA and it looks great.
I would try the AA at 2x because AA makes ALL THE DIFFERENCE visually.
Why the hell are you playing Crysis on medium with 8800GTS 512? Turn off the AA and cranck up the settings to high ( of course the resolution is very important on 12x10 u will play high no problem or on 1440x900) I mean the 8800GTS 512 is still a very good GPU
I hate the AA in Crysis because it makes the vegetation rubbish( too sharp ), and 2xAA makes NO visual difference, if you can put 4xAA then it makes difference.