I’ve had to do this twice in the past week, so I thought I’d pass along how to do it. The majority of this was posted by some guy going by craigevil on some random linux board. Worked like a charm, thanks Craig.

1. Add the unstable repositories to your source list. Open your source list by running

sudo leafpad /etc/apt/sou*

and then add

deb http://http.us.debian.org/debian/ unstable main contrib non-free
deb-src http://http.us.debian.org/debian/ unstable main contrib non-free

2. run the following in order

apt-get update
apt-get -t unstable install nvidia-kernel-source
m-a a-i nvidia-kernel-source
apt-get -t unstable install nvidia-glx
apt-get install nvidia-xconfig
nvidia-xconfig

3. Open your source list again, and comment out the unstable repositories

#deb http://http.us.debian.org/debian/ unstable main contrib non-free
#deb-src http://http.us.debian.org/debian/ unstable main contrib non-free

otherwise the next time you run an update, you might upgrade from Squeeze to Sid. Reboot, and you’re done (hopefully).