ATI can do maximum 16x Anisotropic Filtering and 6x FSAA.
Even though NVIDIA's 8x FSAA is not really better than ATI's 6x FSAA, it still exists and hits performance very tangibly. That is why we stick to 4x - available both on NVIDIA and ATI hardware. The same applies to anisotropic filtering.
Maybe, we will benchmark NVIDIA's 8x FSAA and 8x AF, but I am not sure when if we do.