I also tried 8xAA with 16AF from 4xAA and i lost only 2 or 3 fps. down from 28fps to like 25 or 26fps.
all settings on and very high, without Vsync, at 1600*1200 res, using the most demanding benchmark which is called Concrete Jungle.
somewhere on the net i read that the GTX 4xxx series really...