You need to add more modes to your xorg.conf (/etc/x11/xorg.conf).
From the link above, the area to add more resolutions in would be under your monitor's config options like so:
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1280x1024"
EndSubSection
EndSection
Then the options will show up in the GUI to change them (I'm not sure if that's immediate or you have to restart X to do it, though).
Edit: About drivers, you should be able to do this all purely with the vesa driver. The Ati driver *shouldn't* matter I don't think, but it's certainly possible although I've never experienced it.