No.
Running Crysis is 64 bit mode will not get you more Frames. Its been bench marked a thousand times, all it does is quickens load times and generally seems to smooth out some of the suttering. I too was expecting a big bump when I switched, on account of having 8 gigs of ram. No difference in FPS, but weirdly it seems... "snappier" if that makes any sense. This makes sense given that there were no optimizations made for the 64 bit version of the game as stated by the Crysis devs.