How do I remove Nvidia drivers and revert to nouveau (12.04)?

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http://askubuntu.com – I'd like to give the new nouveau drivers a try. When I try a Ubuntu live distro, they work pretty flawlessly, but I don't seem to be able to use them correctly on my installed system. What I do is remove Nvidia drivers from the Additional drivers utility and reboot, but what I get is no 3D acceleration and low resolution on my monitor. What am I doing wrong? I wonder if I might have broken something when I messed up with grub trying to fix the infamous plymouth splash screen... Thanks for your support (HowTos)