In general, you will need to turn on hardware PhysX in a PhysX supported game for GPU PhysX to be turned on. Crysis does not support PhysX, so that is not the issue with your framerates. The big GPU PhysX games right now are Mafia 2, Metro 2033, Dark Void, Unreal Tournament III, and Batman: Arkham Asylum, Cryostasis, Sacred 2, Mirror's Edge, and Tom Clancy's Ghost Recon Advanced Warfighter 2.