You were probably very close before.
But seeing as you reinstalled (NOTE: this is not Windows -- reinstallation is very rarely needed...), the opportunity to fix it is gone.
What I'd do is boot to runlevel 3, run the installer, change the config file, startx, and make sure that all works.
Then reboot (to clear out the kernel modules; cleared modules is what I'm guessing your problem was -- seeing as we don't have an X log to go from, though, my guess is as good as anyone else's...), still to runlevel 3, and try to startx again. It sounds like you know your way around the shell, so it shouldn't be too hard to save the log file that's probably going to fail.
If it fails, /sbin/modprobe nvidia as root, and try again. It should work. Once you've verified that that's the problem (the kernel module not loading on boot), you can add an alias to /etc/modules.conf (or add "nvidia" to /etc/modules, either way):
alias /dev/nvidia* nvidia
If you edit modules.conf, then run /sbin/depmod -ae as root afterward.