Or, depending on the distro you're using, you can download the drivers directly from ati.amd.com or by the distro's package manager (gentoo with portage has them on their own repositories according to your system flags, but ubuntu makes you download them).
You have to follow the install instructions and there is also a list of packages it says has to be installed before you install this driver - namely:
Now I could never find and install the XFree68-Mesa-libGL or XFree86-libs but they seemed to be installed under slightly different names. Check your Synaptic Package Manager under System/Administration for the other packages. Then follow the installer instructions:
1. Launch the Terminal Application/Window and navigate to the ATI Proprietary Linux
2. Enter the command sh ./ati-driver-installer-10-2-x86.x86_64.run to launch the ATI
Proprietary Linux driver installer.
BUT I had to put the sudo command before the sh ./ati-driver-installer(etc) to make it work in the command shell. Also note that if you look for the file with the dir or ls command only it won't show up because it is hidden - so just type the command as listed in #2 above but with the sudo command in front.
Clear as mud? It took me a while to get it working but finally it does. Also just to note - once it installs you can change settings via System/Preferences/ATI Catalyst Control Center (Administrative) but each time you change a setting (ie for two monitors) you have to reboot before you change another setting or it doesn't work.