It depends on the architecture of the card and the way AA is implemented. In DX10.1, AA is implemented through the shaders (which is a ton of power in ATI cards, hence why they like 10.1). The old method (DX10 and before) is to have a separate unit on the back to perform these functions. If this secondary processor is not large enough, a huge FPS reduction occurs when AA is turned on (example, the 2900). If this processor is large, then the impact is smaller or negligible (example, the 8800).