ATI is sometimes a PITA...
Make sure you installed kernel-source, gcc and make packages.
Then reinstall the drivers.
Then run sax2 as following:
This tells sax2 to use the ati proprietary driver (the module is called "fglrx", don't ask me why did the ati developers choose such a weird name) to configure your X server.
Configure your monitor, then quit sax2 and restart X (relogin).
Don't press "test" button in sax2, this will lead to graphical corruptions or crashes (it's a bug in the ATI drivers)