
Nvidia's proprietary libglx module is incompatible to the standard libglx module which is needed for free and open-source drivers such as nouveau. Warning for older releases (before Oneric?) You don't need to uninstall xserver-xorg-video-nouveau. Restore your backup copy of nf, or edit it to change the Driver from "nouveau" to "nvidia". Click "NVIDIA accelerated graphics driver (version current)". Go to System Settings / Additional Drivers. To revert these changes, just do the opposite. The minimal /etc/X11/nf file, which you should try, if you encounter any problems, is: Section "Device"

Edit /etc/X11/nf to change the Driver from "nvidia" to "nouveau". With all of these proprietary drivers deactivated and the xserver-xorg-video-nouveau package installed, it will automatically use nouveau.

Click the activated driver, which is probably "NVIDIA accelerated graphics driver (version current)". To switch to nouveau, go to System Settings / Additional Drivers. It won't uninstall the proprietary nvidia drivers. Install the xserver-xorg-video-nouveau package. Note: For hints for building nouveau on Ubuntu, see UbuntuTips Ubuntu 11.10 Oneric
