Have you used any NO-CD patches from any Third Party?
If you haven't and HAVE legally bought the games, then you need to clean the rig of AMD drivers, delete registry entries for AMD and then First install the drivers for the new nvidia card from the Nvidia website Not Beta Drivers but the last stable version, and after that install the drivers needed , if any, by the AMD Processor....
Use this :
http://www.guru3d.com/files-details/display-driver-uninstaller-download.html