RX580 will allow you to run medium/high. You're going to have more of a "bottleneck" from your i5-4570.
An easy oversimplification:
- CPU determines maximum frame rate - Achieved at lowest possible game quality settings.
- GPU determines maximum visual quality - higher in-game quality settings at playable FPS.
The CPU processes the game world and tells the GPU what to draw in a single frame. Then the GPU renders the frame as you see it. If quality settings are comparatively low, it's easier for the GPU to render each frame and the GPU completes each frame faster (and vice versa for higher quality settings). At some point, the GPU can render each frame before the CPU tells it what to draw next. The opposite is also true...