I had previously ditched my old AGP and PCI Nvidia 6-series cads (I needed them for dual displays in Windows, but the dual displays never worked right in Ubuntu) since my replacement motherboard had no AGP slots. So I had to go with the onboard Intel GMA X4500HD, which worked like a charm under Ubuntu 12.04.
Do note, both the nouveau and proprietary drivers worked like a charm in single-display mode in both Ubuntu 11.04 and 11.10 (and their derivatives)... basically using the newest drivers avaible in the repos.
Now, I've installed a much more powerful dual-head MSI NVidia GTX 560, and am running Ubuntu 12.04. Now, I am unable to boot Ubuntu. I get a messed up screen, and nothing happens after that. The kernel should have the nouveau drivers built in, and it should be booting with the nouveau drivers.
On top of that, I have a non-persistent Ubuntu 12.10 beta and Kubuntu 12.10 beta LiveUSB's, and neither of those will boot up either. Again, the built-in nouveau drivers should be working even for the LiveUSB's.
Any suggestions other than nomodeset?
I'm trying a few other things, more updates to come.
Updates: Running Ubuntu 12.04 in LiveUSB doesn't boot either. It won't go past the Unetbootin screen. I also have Ubuntu 11.10 on a DVD, and running it in LiveDVD mode doesn't boot up either. It freezes in the middle.