First, look through your /etc/X11/xorg.conf file (you can page through it with the command "less /etc/X11/xorg.conf"). Check the "Device" section: it should say "Driver nvidia" as a start. To check if 3d acceleration is working, install the "mesa-utils" package, and then enter this command "glxinfo | grep rendering". You want to see "direct rendering: Yes". Here's a pretty comprehensive page on Nvidia and Debian:
http://wiki.debian.org/NvidiaGraphicsDrivers
