1. Hit Ctrl-Alt-F1 and login as root from the text console.
2. dpkg-reconfigure xserver-xfree86
3. When you get to the monitor config, you can either specify exact monitor frequencies with the "Advanced" setting or (I recommend) just specify the resolution and refresh rate you want X to use with the Normal settings control.
4. Save and logout
5. Ctrl-Alt-F7 to return to the GUI login
6. Hit Ctrl-Alt-Backspace to restart X with the new settings