Best thing to do would be to switch to the "Standard VGA" drivers that come with windows and reboot a couple times with it before you swap cards. Be sure to uninstall any card specific software as well.
Also, if you are upgrading from a 3dfx card to something else, be sure to delete all the 3dfx???.?? files and glide files. If its an ATI card the files are usually ati???.?? I'm not sure how NVIDIA files are named.