I'm hoping this will also hold for Ubuntu. I had to replace my Mobo recently and needed to rescue my data while using Ubuntu 10.04LTS... Support for which ran out in November while I was sourcing the Mobo
so I also needed to upgrade, in this case to 12.04LTS ( one step at a time!) I needed a minimal system and had to use the onboard video to rescue my info and also upgraded in this configuration but left the original config flies, etc. I also wasn't sure the video card was OK after my Mobo failure.
Replacing the card, an HP badged PCI-e Nvidia Quadro NVS 285 dual headed thingy, The system booted OK with POST info on both screens so the card's looking fine. It only got to a CLI boot screen though and a "startx" command got me flashing cursors in the top left of the screens.
I haven't found anything definite in /var/log/messages, dmesg or any of the other logs regarding the xserver but had come to the conclusion that I wanted to disable the Nvidia drivers and use the generic ones instead, (The system boots fine on a live Ubuntu 12.04 CD)
I'm hoping your post will help, it only appeared on my phone this morning! I'll let you know,