If you want to install NVIDIA proprietary driver. First of all see if you don't have any other version installed. Using Ubuntu go to: System - Administration - Hardware Drivers. Disable it if you want.
To verify the newest version http://www.nvnews.net/vbulletin/showthread.php?t=122606
1. Download the version that you want to install.
2. Access console pressing ctrl+alt+f1 (Don't do it know!)
3. $ cd /download/place/
4. Close the GDM (Gnome Desktop Manager) using: sudo /etc/init.d/gdm stop
5. Run the installer: $ sudo sh ./Nxxx.run (maybe you will need to use chmod +x Nxxx.run)
6. During the installation choose "Update xorg.conf automaticlly..
7. Reboot your system: sudo reboot
In the portuguese version this is almost a translation of http://ubuntuforums.org/showthread.php?t=990978