Configure the X server

Open:

sudo gedit /etc/X11/xorg.conf


Under Section "Module", make sure that the following lines are included, and add them if they are not:

Load "dri"
Load "dbe"
Load "glx"


For an Nvidia card: Make sure the "DefaultDepth" option in the Screen section is set to 24 - the nVidia driver doesn't do argb-with-glx at any other depth.

    (I needed to add Option "AddARGBGLXVisuals" "True" to the Device section as well, otherwise I got "no GLXFBConfig for depth 32" errors. This is on a GeForce4 MX 440, also needed on GeForce FX 5600). 

And finally, add the following to the bottom of the file, unless it already exists elsewhere:

Section "DRI"
        Mode 0666
EndSection


[edit]
Restart the X server

Ubuntu users:

sudo /etc/init.d/gdm restart

If you get a black screen with text and the computer doesn't seem to do anything, hit CTRL+ALT+F1. This will swicth you to a terminal. Log in and then run the "sudo /etc/init.d/gdm restart" command again. It should restart the X server correctly this time and you should land on the GUI login screen. It should be the same for KDE users. Just use the "sudo /etc/init.d/kdm restart" command instead.

Kubuntu users:

sudo /etc/init.d/kdm restart