AA kills Crysis. Instead, open the console and type "UseEdgeAA 2" without the quotes. That will give you an AA-like effect that even works on the foliage (something that normal AA doesn't do). This "edgeAA" is not compatible with normal AA, so make sure you disable that or you won't see any difference.
Oh and just in case you aren't familiar with the console, access in by pressing the ` key (to the left of 1 and under Esc). To unrestrict it you must type "con_restricted 0", again without the quotes. Then do the little edgeAA command.
Hope that helps