You could try using envy. To install in a terminal run
sudo apt-get install envyng-gtk
or look for envyng-gtk in synaptic. Then you will need to run in a terminal
it will give you a option to install ati drivers select the corresponding number. Then it will give you a list of the most compatible drivers to use with your card select the number here to. I believe envy will ask if you want you xorg.conf file changed select yes. That should do it, and then reboot.
I attached two images of what envy looks like although I am using nvidia it gives you an example. I am assuming you are using Ubuntu 8.04 or newer. If not you need the envy legacy package. This is the website for envy
there is quite a bit of good info there. good luck