My friend has an amd motherboard/amd phenom 2 x4 965 CPU. He has a radeon card at the moment and he purchased a gtx 660 SC, it's in his hands. I told him to simply uninstall the drivers and reboot, install the new nvidia drivers with his new card.
However, every guide we can see on the internet says this:
"WARNING: Do not use this method if you have an AMD processor/chipset. This guide is only for users who have Intel processors and AMD graphics. Using this method on an AMD chipset will cause vital chipset drivers to be deleted and create system instability."
"Do not use this guide if you have an AMD CPU/Motherboard as you may destroy required chipset files."