I'm willing to bet you haven't put the nvidia module entry into /etc/modules. Aieyah. You have to do this for it to be loaded at boot time.
# /etc/modules: kernel modules to load at boot time.
# This file should contain the names of kernel modules that are
# to be loaded at boot time, one per line. Comments begin with
# a "#", and everything on the line after them are ignored.
# NVIDIA driver
The error message you provided was hardly very specific, X gives that message for most nasty problems. Usually the thing that went wrong is just above that. Giving the output of "tail /var/log/XFree86.0.log" would be a bit more helpful.