So you are saying that Xserver won't start? That's what it sounds like to me.
If this is so, there should be a log file saying what went wrong. More than likely something mest up when it installed the video driver. What you may want to try is logging in as root, at the command line, and re-installing the RPM ATI driver, or un-insalling it. (rpm -e ati-driver-package-name).
Sorry this probably isn't too helpful.