AF is ansiotropic filtering, which is much less demanding than AA. AA is antialiasing, which is VERY demanding on your GPU. For instance, in Doom 3, at no AA, i get an average of 80fps on the time demo. But if I just enable 2x AA, that gets cut in half to near 40 fps. 4x AA puts me down in the high 20s. I see the same thing in Far Cry. You only see slight performance decreases from AF though. AF makes textures in the distance appear clearer.