Dr. Nikolaus Klepp wrote:
Usually it's selfconfigured, but you could check if /etc/X11/xorg.conf or /etc/X11/xorg.conf.d exist. If yes, it might be a good idea to delete these and try again. If they don't exist, you'll have to ask the searchengine of your trust (sorry, don't have any ATI GPU here)
AMD discontinued support for older cards and firmware might not work is what I know. I experienced this on one ubuntu, but might be something else - worth checking.