X is the graphics subsystem that Linux uses, and the X Server is the program that handles drwaing the screen. To shut down X press, ctrl+alt+backspace, but X will automatically restart unless you tell it not to. To disable automatic restarts, in a terminal run:
Once X is disabled, you'll want to restart it again when you're done, to get X to automatically restart again:
sudo /etc/init.d/gdm stop
All of the above is the hard way to install the nvidia driver. If you want a tested stable driver the Hardware Driver manager at System > Administration > Hardware Drivers is much easier. If you want the newest drivers, install envyNG and have it install the drivers for you.
sudo /etc/init.d/gdm start