AMD R6-series hardware supports 4x MSAA as the base level (well, 2x, but PC graphics reasonably ignores a two tap AA filter these days, so we do too), with 8x the highest sample count, improving on the 6x of their last hardware generation.
The grid shows good sample distribution for 8x, although it does bias the top-right and center of the subpixel at first glance. We've yet to see EA release Very Long Thin Triangle Sports 2010 - Even At Really High Res Edition, yet, so we think AMD are probably safe.
As you can see, the custom filter modes are set using a simple control panel interface. You choose a filter and base MSAA level and the final tap count becomes a function of those two settings. Edge detect can also be thrown into the mix in more recent drivers, improving image quality even further.
Page 14 of our initial analysis covers things with theoreticals, page 15 looks at in-game AA shots. You'll get further links to output on other hardware as well for comparison, on that same page 14. Anisotropic filtering is next, and then we'll wander into the aforementioned chatter about downfilter routines and their effect on image quality.