I'm far away from this computer, so
First, switch to a console (Ctrl-Alt-1)
First, slay gdm (hard when you can't see the pid using ps -A, so redirect
the output to a file and use vi to fiddle with it)
Then run Xorg -configure
Edit the ~/xorg.conf.new
Fix the HorizSync and VertRefresh under the Monitors section
Disable Hardware Acceleration
Test the new configuration using Xorg -config ~/xorg.conf.new
Replace your real xorg.conf with ~/xorg.conf.new
You should have a nice, non crashing X Server with Gnome Desktop.
