I've been using the in-game x4 fsaa.
I just tried The Witcher for a bit with this card and lookind at onscreen FRAPS numbers while in combat with three ghouls:
x4 in-game: hovered in the 90-130fps range with a couple of fast dips down to 76fps
16xQ: Mainly stayed in the 90-111fps range but there were a couple of split second dips down to a low of 39fps and one "hover" period in the ~55fps range that lasted a cuple of seconds.
I guess with the lowest dip down to 39fps, I should leave 16xQ on.
Wow that looked good! Even x4 in-game had a much better visual appearance than on the 8800GTS. Most cool. I'm a very happy camper today.