You need to remove the drivers.
So if it's anything like the remiveal of the nvidia drivers then
sudo apt-get remove "whatever the ATI driver package is"
Then you need probably need to edit your X config.
Here is a guide to install many idfferent packages including the ATI drivers. It might be able to help you.
http://easylinux.info/wiki/Ubuntu_dapper
http://easylinux.info/wiki/Ubuntu_da...iver_.28ATI.29