BOAC,
With Windows and video cards it is always preferrable to remove all previous drivers before installing a new card.
First go to the Add/Remove Programs Control and remove everything related to your video card in there. Reboot when asked to, and if you aren't asked to, do it anyways. Don't know about S3.
To make sure you have removed everything you can look for a program called Driver Cleaner which should do a more thorough job in the case of some drivers. I have heard that the nVidia drivers are bad for leaving behind the odd file or registry setting (I don't have any expereince with recent nVidia cards though). ATI has an uninstall program of their own called "Cat-Uninstaller" which removes all ATI software on the computer.
When the old drivers have been removed you can install the new card and drivers. It's a good idea to do this when you update drivers for the video card too to reduce the chance of conflicts between driver versions.
Hopefully this will help.
goates