As mentioned, uninstall the AMD drivers, install new card, install new drivers. If you have a folder; C:\AMD\AMD_Catalyst_<insert version umber> (ex; AMD_Catalyst_13.11_BetaV9.2) you can go in there and run 'setup' to uninstall all AMD related software/drivers.
If you're on Windows 7 then it's not required to uninstall drivers and you could just pop in the new card and install Nvidia Drivers, but it could potentially cause issues later. I personally run both Nvidia and AMD drivers at the same time with 0 issues.