ATI is not a Linux friendly company. Most Linux users migrate towards Nvidia
when they can. You have stumbled on one of the great mysteries of computing.
Some hardware does not match up with some distros, but another one may be just
the ticket. There is no rule and no one best distro. The best one is the one
that works for you. The important thing to do is to shop around and be
persistent. Live CDs help with the process,