The ATI 9000 pro can go up to 2048x1536 res according to the manual
here. Just make sure your monitor supports whatever res you want to use and then update your xorg/xfree conf file with the res. You will definitly need to install the ATI drivers to get anything higher than 1024x768 I believe (good luck with that, heard it's real fun). You can get the driver
here.