I had the same problem in my 6-core, Radeon 6770 4GB Ubuntu 11.10 box, although HDR always was better than X-Plane on Win7. Switched to Gnome Classic without effects (no Compiz) and AA started working. Now I get twice the fps in Linux, so happy that I'm actually considering not booting Win7 again (wouldn't be able to play MW3 though...) :)