Seems to me option for quad-AFR is go use them not for serial framerate increases, but to increase AA or add temporal AA. Another option is maybe "speculative AFR" in other words, if the game can determine that based on inputs (controls, network) that the game state has updated, but not in a way that invalidates the N+2 or N+3th frame significantly, then it can use AFR, otherwise, it would discard those frames. Of course, the only problem is, this would make framerate jerky, as well as fall down where framerate falls down the most - rotating the view.
Given a quad-7900GTX, you might be able to get away with 16x temporal antialiasing on older titles. Each card could render the frame 4 times, and with 4 cards, you've get 16x.