First you need to do these things before installing propietary driver:
1. Uninstall nouveau.(aptitude purge xserver-xorg-video-nouveau).
2. Blacklist nouveau and possible Framebuffer drivers for nvidia card. (cat /etc/modprobe.d/blacklist-framebuffer.conf) and see if there are banned/blacklisted (nvidiafb and rivafb), if not, do this:
2.1 open the file: nano /etc/modprobe.d/blacklist-framebuffer.conf and add them: blacklist nvidiafb and blacklist rivafb in separate rows. just like the rest. Ctl+o to save and then Ctl+x to exit..
Then you bootup and enter recovery mode, once in text mode (terminal), go to the location of the nvidia driver downloaded and install it. After that reboot.
If you had uninstall and blacklist after startx, you could have gone to text mode, kill the X's and install priopetary driver from nvidia and restart the X's...
Now you have to uninstall nouveau and blacklist if needed....Then reboot.
Any doubt, let us know...