Stopping the X server
In gnome, I had to stop the X server once to install the nvidia drivers. So here's the best way to do it.
While in gnome, hit the key combination ctrl + alt + F1 and that will take you into text mode, then login if you have to and,
# /etc/init.d/xdm stop
Do whatever work you need to do here and then,
# /etc/init.d/xdm start
This should take you back to the gnome login screen.