ATI does not support Linux.

From their web site

Operating Systems Support
Windows® XP Media Center Edition 2005
Windows® XP Home Edition
Windows® XP Professional Edition

As far as I know this is not an Ubantu problem but a problem with ATI not keeping up with the times and not supporting an up and coming and very popular operating system like Linux and a reason why so many people still use Windows.

I have searched and found no third party drivers for ATI drivers but there should be a default driver that should give you decent resolution no matter what card you are using.

Also since I expect you are not going to be gaming in Linux you may want to see if your computer\'s motherboard has onboard graphics. You can then turn that on in the control panel before the computer boots and use that as your primary graphics source in Linux and use your ATI card in Windows.