When Ubuntu is loaded it will have a configuration file called /etc/X11/xorg.conf created. Usually it is a matter just edit this file.
My suggestion is to try drivers "ati" or "radeon" in the line
where ??? is the driver Ubuntu has chosen for your PC.
Generally a user can edit /etc/X11/xorg.conf to get the monitor and video card to match.
Linux uses generic drivers that work in the majority cases. I have installed a large number of them and this how I got by. Have two versions of ubuntu running in my machine too.