You are correct, X is trying to use the nvidia drivers. Ubuntu does not bother to autodetect the videocard every time you boot up, so it assumes that the nvidia card is still in place.
When this fails, X returns an error, some info to help (which it did), and gives you a simple terminal to use to fix this.
The best fix is to get X to reconfigure for the new card. You do this my entering the following command in the terminal:
sudo dpkg-reconfigure xserver-xorg
... follow the instructions.
... and that is it.
If you want to explore further, the video and associated x server configuration lives in a plain text files /etc/X11/xorg.conf ... it is good to look at this as it usually contains a whole load of stuff you don't need.
Last edited by Simon Bridge; 02-13-2007 at 12:09 AM.