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# stop gdm
/etc/init.d/gdm stop
# The Debian and only way to do this 
apt-get update && apt-get install nvidia-kernel-source module-assistant nvidia-settings nvidia-xconfig nvidia-glx
# to be sure
modprobe nvidia
# reconfigure automatically your graphic card
dpkg-reconfigure xserver-xorg
# restart gdm
/etc/init.d/gdm start