What is your resolution, screen size and level of AA? Running 2x AA but only having a 1024x768 resolution on a 20" monitor is pointless. Basically you should only be using AA, especially in Crysis, if you are running at a higher native resolution (1680x1050 and higher) and can afford the performance drop you will take for AA. It does make the game look better, but for Crysis it's really a luxury if your system can afford it.