I installed the ati drivers
Buy a Nvidia card... seriously I changed back to Nvidia from a failry good radeon due to ATI's complete lack
of usability in any OS other that microsofts I was messing round with thier proprietry drivers and the free
ones for 2 weeks and in the end said my time was more valuable and I had bought a new card 4 times over
in the hours I had spent on it. Nvidias drivers will *generally* work right out of the box for all distro's ATI only
support the corp distros i.e. RH, Suse (now owned by Novell), and some "limited" support for debian.
But if that is not an option ATM try and make sure your card is configured correctly. see
ATI's Support section for linux