Screen corruption on Unity with GeForce 6150

I imagine you're running Ubuntu 11.04 Natty (which is still in beta, and with which you should still expect significant problems)? If so, then the bad behavior and poor performance of the Unity desktop session is likely due to a bug in compiz--I recommend submitting a bug report. To do this, first read https://help.ubuntu.com/community/ReportingBugs carefully. Then, you can report the bug by entering a Unity desktop session and running ubuntu-bug with the PID of the currently running compiz process. This will make sense after you have read https://help.ubuntu.com/community/ReportingBugs. (Since this desktop environment is very difficult to use, I recommend that when the web browser comes up with the Launchpad window, you immediately bookmark the page, then log out, log back in with a classic desktop, and load the bookmarked page--Launchpad should still remember the association between this URL and the information uploaded by Apport. Alternatively, you could run apport-cli instead of ubuntu-bug, and get the URL without ever loading the web browser.)

In case this turns out to be a bug in the proprietary NVidia drivers rather than compiz, after you submit your bug report I recommend you use the "Also affects distribution" link to mark it as also affecting the package nvidia-current in Ubuntu. This (or, less likely, compiz) will likely be marked Invalid, but the other package will remain and this will speed up the process of finding out if this bug is a duplicate of other bugs (or if other bugs are duplicates of this bug). It's a good idea to express your uncertainty about what package is properly affected by this bug in the bug report itself, to motivate people with the necessary knowledge to consider if it is properly classified.

Also after submitting the bug report, you can link it and this question page together by using the "Link existing bug" link near the top of this question page. While that's a good thing to do, your bug report should nonetheless be complete and self-contained--it should not be necessary for Ubuntu developers to refer to this question page in order to fully understand and appreciate your bug report.

The problem starting up Ubuntu with the nouveau driver enabled is almost certainly also a bug, though the nature of this bug depends on precisely how your computer is unable to boot. Assuming you're running Ubuntu 11.04 Natty, Apport should be configured to automatically collect information about crashes and other abnormal program terminations or kernel panics, and offer to report them. So perhaps, after an unsuccessful attempt to boot Ubuntu with the nouveau driver enabled, you are offered an opportunity to report that as a bug. If so, the best thing to do is to report it in that way.

Otherwise, however, it would be useful to have more information, in order to figure out exactly what package it affects (though this is almost certainly xserver-xorg-video-nouveau) and what the bug consists of. When you enable the nouveau driver and reboot, does the system freeze when you see the Ubuntu logo in the middle of the screen, or does it freeze after that while attempting to start up, or does it give you a blank (or perhaps purple-tinted) screen, or does it give you a text-based (i.e., non-graphical) login screen?