There's a howto on installing the ATI drivers over in the Hoary Tips and Tricks section of the forum.
If your card is a bit older (easy to install):
http://www.ubuntuforums.org/showthread.php?t=24557
If your card is one of the new ones (I believe X600 or above model # wise), or you just want the newest drivers you can get (a little harder to install):
http://www.ubuntuforums.org/showthread.php?t=32495
Also, if you want to get into a GUI, you can use the generic video driver. Boot into recovery mode from the grub prompt, and enter:
Code:
sudo nano /etc/X11/xorg.conf
Scroll down to the Device section and replace the driver (which will be ati or radeon) with "vesa". Then type CTRL-X to exit (hit y to save your changes). Then type "reboot". You should have a GUI, albeit a bad one.