Make sure you have back-ports repository configured if you want a more recent version of the nvidia drivers which i highly recommend. Also the steps below need to be followed exactly so proper dependencies are met.
This will install kernel headers from stable
Step 1: apt-get install linux-headers-`uname -r`
This will pull in a more recent version of the Nvidia Driver from backports
Step 2: apt-get install -t squeeze-backports nvidia-glx nvidia-xconfig nvidia-kernel-dkms nvidia-settings
Step 3: nvidia-xconfig