Ah yesss...
Crysis.
Crysis will do that to systems....
Check any background apps/programs running and switch off if not needed, check task mger for that too....
you can switch off your anti-virus whilst playing, anything to give you some extra "juice" is what i'm indicating here...
maybe crank down your screen resolution a notch or two, you can pick up anywhere btw 2 to 10 fps....
check out the forums for Crysis, might be an answer there for you:
http://www.incrysis.com/forums/viewforum.php?id=26