I just tried installing nvidia legacy drivers for an 8800 GT and blacklisted noveau, after rebooting the system, an error screen would appear saying that there was an error and I should log out and try again.
In that very moment I realized that there was a conflict because of the non existence of the Xorg.config file, the presence of the nvidia drivers and the noveau drivers being blacklisted. So in order to be able to log in again I had to delete all nvidia related files and un-blacklist noveau.

While on the error screen press Ctrl + Alt + F2 keys combination.
Then,
This should purge all nvidia files
Code:
#sudo apt-get remove --purge nvidia-*
If there is an error run this:
Code:
#sudo dpkg --configure -a
Then we force load the noveau driver on the next reboot
Code:
#echo 'noveau' | sudo tee -a /etc/modules
Then reboot the system and you should be able to normally boot.