Try an /sbin/modprobe NVdriver as root first (this loads the nVidia kernel module). Then see if you can run X. The problem with this fix is that it doesn't fix anything after a reboot, only until you shut down.
If you can run X, then make the change persistent by editing /etc/modules.conf, and adding a line looking like this:
alias /dev/nvidia* NVdriver
Save and exit, and run /sbin/depmod -a to update the module dependencies (this has to be run every time you edit /etc/modules.conf, otherwise you'll get warnings when you try to load modules -- not a huge deal, but it's easy enough to fix). Once you do this, you should be able to reboot, and X should load fine after that.