First, you disable the onboard VGA in the device manager. Then you have to completely remove the ATi drivers with this tool:
http://downloads.guru3d.com/download.php?det=745
You must do that before you install your nVIDIA or the drivers will fight with each other. You can't even really use the uninstall feature in the Control Panel because tidbits are still left behind.
If you were told anything else, ignore it.