Type this into a console of some sort, to begin with:
[ -c /dev/.devfsd ] && echo "devfs"
If it prints devfs at you, then go with line 1 below. Otherwise, go with line 2.
Add one of the following lines to your /etc/modules.conf file (this assumes you aren't using Debian... if you are, then there's another file you edit, but I don't know what it is).
Line 1: alias /dev/nvidia* NVdriver
Line 2: alias char-major-195 NVdriver
The difference is the way the old device system worked versus the way the new one (devfs) works.
Anyway, now run depmod -a as root (actually, you should probably do all this as root), and it should work. You can modprobe -r NVdriver (as root) to remove the driver, then try to startx again (as your normal user), to see if it worked. Or you can just reboot.