Not sure if this is a bug, but I'm able to select 16x Antialialising in the graphics options. I own a ATI5870 and Catalyst Control Center only offers 8x AI ... ? Is the max. AI-level read from the graphics adapter or maybe just a fixed value?
It is a fixed value. Then internally it will automatically select a lower setting if not supported by the hardware.
It run quite fine with 16x in a Nvidia Gtx 285!
You can try to force 16x through the catalyst software, look for update your drivers.