Also I have an nVidia display driver. They says that to install, type "sh <filename>" after terminating X server. How can I terminate X server?
I assume you get a graphical login screen. If so, type
at a terminal prompt and you'll be at a plain text console with no X running. I don't *think* you need to be root to run that command, but I'm not sure. To get back into X, type
at the command prompt.
I prefer runlevel 2, so I can terminate X simply by logging out of my XFce desktop environment. But for runlevels 4 and 5, if you log out, it puts you back into X immediately.