Hello, I just retrieved the information for both cards you mentioned above from Wikipedia, and re-paste it below
From the aspects of Processing Power, AMD RX 480 is much stronger than NVidia GTX 770. I have never used RX 480, but I guess its performance might be at the same level as Geforce GTX 980 or 1080, or close to the later. Maybe that is the problem of GPU drivers, would you please remove both drivers, and clean them up. Then reinstall the latest AMD Crimson driver for your card! I wish you good luck!