The RX 580 is just a rebadged RX 480 with a slight boost. You would be buying the same performance as you already have.
Edit - Also, if you are looking to get an 8GB model to replace a 4GB model, you still only gain maybe a 10% increase in peformance. You would need to get an AMD Vega56/RX 5700 or Nvidia GTX 1070ti/1660ti/RTX 2060 for around $250-350 to see an actual improvement over an RX 480.