NVidia drivers

You have to shut down X server in order to proceed with installation.

I too am having a problem with this -- already had to reinstall Ubuntu to get out of it. As i interpret the documentation for my (legacy for a Viper 770) driver and the Ubuntu help files, it MIGHT go like this.
  1) install the correct driver package using the Synaptic Package Manager.
  2) use sudoedit in a console window to alter the file /etc/X11/xorg.config like so:
      A) in the "Module" section, make sure this line is present:
             Load "glx"
          also remove the following two lines if present:
             Load "dri"
             Load "GLcore"
      B) go down to the "Driver" section and change the line
            Device "nv" to
            Device "nvidia"
  3) then to update the checksum, run the line:
      md5sum /etc/X11/xorg.conf | sudo tee
      /var/lib/x11/xorg.conf.md5sum
          ( all in one line and mind the capitals )
  4) then, it should be possible to run:
       sudo nvidia-glx-config enable
Up to now this command has produced an error stating that the X-configuration had been altered.

I have two questions for those who know more about this than I do:
    1) is the above the correct sequence -- if not, where should it be corrected?
    2) if it necessary to shut down the X server, how and when should it be done?

tcg

X server should be shut down before attempting to install nVIdia driver setup.
You have to move to a virtual console pressing CTRL+ALT+F[num], where [num] usually goes from 1 to 6.
After you login, type sudo -i in order to get root privileges and type /etc/init.d/gdm stop. This stops X server and now you are able to launch nVidia driver setup.

I was able to install the driver for my (Nvidia) Viper 770 video card, and my experiences may help others. I followed this sequence.

    1) install the driver package for the type of card you have -- mine was the "glx-legacy" version. A newer card will use the standard "glx" driver.
    2) as described above by Luca Falavigna , go to a virtual console and shut down the X server.
    3) run "sudo nvidia-glx-config enable". If you get an error message (as I did) stating that a file has been changed, the checksum has to be updated. Run:
      md5sum /etc/X11/xorg.conf | sudo tee /var/lib/x11/xorg.conf.md5sum Then run sudo nvidia-glx-config enable once again and it should work correctly.
    4) VERY IMPORTANT!! Somehow the script you just ran has an error in it and the "bus_ID" parameter has been changed. You will not be able to start up the X server. This is the cure:
        A) go to /var/backups/xorg and look for a file with a name like xorg.conf.<year>.<mo>.<day>.<hour>.<min>.<sec> The date will be the exact moment that your backup was made.
       B) copy this file by executing:
         cp xorg.conf.<year>.<mo>.<day>.<hour>.<min>.<sec>
         /etc/X11/xorg.conf (all in one line with your values in the angle brackets)
       C) enter: sudoedit /etc/X11/xorg.conf Slide down to the part that reads Section "Device". The Identifier should describe your video card, and BusID should say "PCI:1:0:0" The Driver parameter will say "nv". Change this to "nvidia".
Make no other changes to this file and save it.
    (5 now you can restart your X server with: /etc/init.d/gdm start
    (6 you can return to the GUI with Crtl/Alt/F7, but your hardware acceleration is still not active, so:
    (7 reboot.
You can then run glxinfo | grep rendering in a non-root command window and you will see direct rendering: Yes

tcg

I can't install the driver (X shut down, on console only) because the setup tells me something about a kernel dev tree or something is missing. On Debian it should be called kernel-devel or kernel-source, it said, so i installed them via apt-get, but the setup shows the same issue...what now? :(