System reinstall after new GPU

Solution
No, that's unnecessary. The only thing you need to do is uninstall any old drivers and use CCleaner to clean anything leftover. Afterwards, just install the drivers for the new card.
No, you don't need to reinstall windows. Make sure the pc is off. Probably safer to have no power turned on but not needed.
Plug in the gpu and then plug the monitor cable to the gpu.
Turn on the pc and install the latest drivers from AMD/Nvidia own website.