GPU accelerated PhysX will decrease your FPS by roughly ~25%, depending on various factors. You can disable it from your game's Settings Menu. Many games feature CPU physics, which generally cannot be turned off. Which game(s) are you having problems with? I'm assuming you are using an Nvidia card?
yes i am using an nVidia card. Mainly, i'm just trying to get as high fps as possible. I don't really understand physx but if a game (crysis) doesn't use physx then would it still be on and eating fps?
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.