I replaced my radeon 7770 with a radeon RX480 GPU and it installed without issues. However, when I try to play previously played games( I only tried 2), One game starts but is so laggy it isn't playable at all, other game says I need DX11 compatible GPU, checked with dxdial.exe utility, which I do have. Other issue, I have 2 monitors, only one comes to life, when I open AMD Radeon Settings, it says I have no AMD Radeon installed. I checked device manager, 480 is there, I clicked update driver through the search online option, it found new driver and installed it properly. I am gonna try to un-install AMD drivers and re-install the driver that came on a disc with RX480.
I have 600 watt PSU.