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#11
You can put a PCIe3.0 GPU in a PCIe2.0 slot, no problems at all. You can also put a PCIe2.0 gpu in a PCIe3.0 slot, no worries there.
I think your issue is that you did not uninstall the previous GPU driver before swapping out the cards. To get rid of all GPU driver traces you need to boot into safe mode and uninstall the old driver from there, and at the same time you should uninstall the new GPU driver too, for a fresh start.
When you are back into normal Windows booting mode, it is very normal that Windows takes it's time to configure things. While this happens, usually Windows is quite unresponsive.