TMS AA, TSS AA, PA AA, QA AA... is transparency anti-aliasing getting you a bit confused? Bit-Tech gives us the lowdown on just what all the acronyms mean, with lots of clear words and illustrative pictures:

Transparency Multisampling:
NVIDIA's transparency anti-aliasing technique is a great feature for mid-range video cards and is also a great feature for high end cards in more intensive games like NFS: Most Wanted, where enabling transparency supersampling has massive performance implications that usually mean lowering the resolution in order to achieve smooth gameplay. However, it doesn't offer the same level of quality that ATI's performance adaptive anti-aliasing algorithm can deliver.

There is little reason not to enable this feature, as it does have a positive effect on image quality with a minimal performance hit on the frame rate. It helps to make alpha tested textures within close proximity of the player much smoother and easier on the eye. The quality of distant textures doesn't seem to be improved quite so dramatically - the minimal performance hit explains that, though.

Performance Adaptive AA:
We believe that ATI is still using a supersampled technique for its performance adaptive anti-aliasing technique because there is evidence that supersampling is being applied to the scene. In order to improve performance when performance adaptive AA is enabled, we believe that ATI is using an adaptive anti-aliasing divider to reduce the number of samples taken by the algorithm.

This is actually a pretty intelligent way to do things, because it means that the quality of the output is higher than what can be achieved with a multisampling algorithm. The downside is that it costs more performance than NVIDIA's transparency multisampled anti-aliasing algorithm, which can almost be enabled for free without any hugely noticeable performance deficit.