Look in your /etc/X11 directory for a file called xorg.conf.old or something similar to indicate a backup file. Rename it to xorg.conf, and the next time you startx it will use that file.
You should be able to make a few minor changes to the backup file to make it use the new drivers. Namely, in the Device section, the driver should be "fglrx" instead of ati or radeon or vga or whatever it was before. Then check that there's a DRI section with 666 permissions (usually at the end of the file) and that the Modules section is loading dri and glx.