upgrade you cpu i5 2500k, then see your performance at ultra.
Dude he does not need i5 2500k for crysis. His i5-2320 is perfectly fine and is in fact a very powerful CPU which will be enough for any modern game.
@OP: It's the graphics card. Crossfiring HD 6950 would be a nice idea if you can get another one relatively cheap (up to $200) and you have a PSU that will support it. Otherwise, I'd suggest GTX 670. It's faster than HD 7950 for the same price.