This might not be the case. Depending on the system, he may already be in runlevel 3, but may be running X. Gentoo Linux, for instance, only uses 1 default runlevel. If you want to boot into a GUI, you just add gdm to your rc.conf
But anyway, back on target.
If changing runlevels doesnt work...
#> ps auxww | less
(X is running somewhere towards the bottom of the list)
get the PID of the X process
#> kill -9 [insert PID here]
that will definitely kill X
then you can go ahead an sh your nvidia installer
once your nvidia installer is done,
make sure you edit your XF86config so that it loads glx, but you disable dri
change your driver from nv to nvidia
and then finish with
#> opengl-update nvidia
This is how I always set up my nvidia drivers.
Hope it helps =)