Something that should be noted WRT to AA modes is the distribution of the sample patterns. While ATI doesn't provide a mode about 6X MSAA mathematically is should provide better edge aliasing over a wider variety of situations than the 8X modes NVIDIA uses - with a sparce sampling pattern none of sampling points intersect with one another either vertically or horizontally, which is not the case with the 6X or 8X modes NVIDIA are currently using.