5870 AA

hey all. i have a ASUS EAH5870/G/ overclocked to 950core and 1300mem, my issue is playing Crysis when adding the AA to 2x up to 8x. i average 50FPS in 1080p everything set to the highest setting and no AA. But as soon as i add just 2x AA my FPS drops to 40 which i think is a bit to far of a drop, then i up it to 8x AA and my FPS is 12. is this a problem with my card? i see this quite often with ATI cards and crysis but i think it has to do with the game. no other game has this issue. but then i put in my GTX480 which is OCed exactly the same to test, i apply the same setting and 16x AA and i still average 50FPS. does crysis's AA function only work correctly on Nvidia cards?