Use a ferramenta Additional Drivers (procure por ela em seu painel) ou baixe os drivers da ATI do site e instale-os com esses comandos
sudo apt-get install build-essential cdbs fakeroot dh-make debhelper debconf libstdc++6 dkms libqtgui4 wget execstack libelfg0 dh-modaliases
sudo apt-get install ia32-libs (for amd64) systems
sh ./ati-driver-installer-11-9-x86.x86_64.run (select build deb packages)
sudo dpkg -i fglrx*.deb
sudo aticonfig --initial -f