Screen resolution messed up after upgrading Ubuntu

I just upgraded to Linux Ubuntu 8.10 (from 8.04) on one of my computers. Under Ubuntu 8.04 I had my screen resolution set at 1280 x 1024.

However, apparently now this resolution is not anymore available to me. The maximum resolution shown in the System -> Preferences -> Screen Resolution window is 1024 x 768.

Any clues, anyone?