An excellent document that covers installation of nVidia drivers on Debian systems can be found here
Also, startx is normally used when you boot in to console mode and would like to start the X Window System. It's normally not used when a Display Manager (XDM, GDM, etc.) is used. If you're using a DM, you should restart X by using the appropriate init script. For example, if you use GDM (typical among recent Debian installations), simply type: "/etc/init.d/gdm restart" (without the quotes); no reboot required. This will start X on vt7, and you can get back to vt's 1-6 a la CTRL-ALT-F1 thru CTRL-ALT-F6.