Sorry to get confusing even more, but is the new card AGP or PCI-e?
If so then you should be able to, I have
Simply boot from the onboard first, and allow XP to install drivers for the AGP/PCI-e Card and you should be away, just remember to make the graphics card monitor primary inside windows tso that it runs all your games through the graphics card rather than the onboard.
This will
not work with just a standard PCI card, something to do with XP and not starting the PCI card at bootup.
Hope this helps