A quick howto for proprietry Graphics card driver install:
First go to "System->Administration->Hardware Drivers", see if it lists a driver for your card in there, if so then click to install.
If that does not work then try using envyng. First install envyng-gtk from the repos:
then run envy (its in one of the applications menus), and install your driver that way.
sudo apt-get install envyng-gtk
If both of those fail then you will have to install the driver manually, which can be a little tricky with ATI, but definately do-able.