Comment 8 for bug 873929

If you're using Oneiric, the only thing you need to do is remove the nvidia_173 or nvidia_173_updates driver.

The command "jockey-text -l" will give you what supplementary drivers are installed, which you can remove with e.g. :
jockey-text -d xorg:nvidia_173

Remove all activated, xorg-prefixed drivers and the system should fall back to nouveau. IMHO nouveau is the best you can get if you don't have a proprietary driver supporting your GPU. If you're a perfectionist, you can even purge the remnants of the nvidia drivers, get a list with :
dpkg -l 'nvidia*'

You will notice some packages have the status flags "rc" which means they're removed/scheduled for removal ("r"), and configuration files remain ("c"). Then do an "apt-get purge" followed by the names of those nvidia "rc" packages separated by spaces... But don't remove nvidia-common !