If something similar to this has been addressed elsewhere, please feel free to point me there. Otherwise, here's the steps I anticipate taking to do this (running Slackware 10.1, BTW)
1) Backup existing xorg.conf file
2) Set machine to boot in runlevel 1.
4) Add "new" card, plug monitor into "new" card (an old Matrox MGA 4MB PCI card...)
5) Restart, run "xorgconfig" as root (assuming no problems in console mode)
6) Try startx
7) If no problems, set machine back to runlevel 3, restart to usual login manager.
8) If any problems, restore old xorg.conf file, remove new card, plug in to old monitor outlet, and come back here
(Just kidding on that last part... I hope...)
Not sure if machine bios can disable onboard video/ enable new video, but I plan to check that as well. Machine is an old Compaq Deskpro (500 MHz Celeron, 256MB Ram, 6 GB HD).
Any suggestions/warnings/"been there-done that" appreciated