This is just on the offchance that somebody does what I’ve just done and upgraded from a desktop Ubuntu 9.04 installation on a Shuttle XPC (this one is a 2006 AMD64 model with motherboard graphics only) that has a 1600 x 1200dpi monitor attached.
After the upgrade you may find you cannot log in using the standard GNOME graphical interface. When you do so, the monitor blanks for a few seconds and then you are returned to the login screen. But at the bottom of the screen is a small menu allowing you to choose whether to log in with regular GNOME, an xterm, or failsafe GNOME. Try failsafe GNOME, and if you can log in successfully use system > preferences > monitors to reduce the screen resolution, then try the regular GNOME login again.
Update I think that I have an i386 install of Ubuntu (not entirely sure why). That might explain why the video driver, which I believe is OpenChrome, won’t load. The integrated chipset in the machine (Shuttle SK21G) is entirely geared to the AMD Sempron, a 64-bit chip. Falling back to the failsafe video driver gets around the problem, but this is obviously far from a good solution :-(