[Solved] Far cry aa laggy

I bought a 4870 1gb from xfx figuring I would be able to play far cry with all settings maxed, seeing on Tom's that the average was 40 fps

but whenever I turn on the aa the game starts getting laggy even though craps shows I'm getting 40 fps, is this somehing that is wrong with the game or does anyone have a similiar problem?

Oh I have checked my processor status while playin and ram status and this is not the problem, oh and my monitor res is 1600x900

ok thanks