Have you done any additional config after install?
It seems like I usually have to boot to a console login (add the number 3 to the end of your grub boot options), and then do some additional config.. when logged in as root at a console promt, try the following commands with and reboot:
aticonfig –initial -f
when video is run you will get the command line version of the video config tool found in PCC.. select the ati-fglrx driver and reply ‘Yes’ when asked to use the proprietary driver. If all goes well you should be able to reboot with working graphics
..if all else fails the install process should have added a fglrx-uninstall.sh script that will return your install to its previous state; I don't recall exactly where it gets installed to, but you should be able to fine it using the locate or find commands.