When you upgrade stay away from older technology like non-DirectX 12 cards. You'll be burned. AMD Polaris hardware (including rx400 series) and Windows 10 software are tightly integrated, so going forward dramatic graphics improvements are coming. DirectX 12 provides deeper access to the GPU than DirectX 11, meaning less overhead and better performance. It also enables a next-gen SLI effect, where two graphics cards are used at the same time. DirectX 12 loosens this up, letting the resources of two completely different cards be used simultaneously.
DirectX 12 enables new shader and rendering techniques for upgrading the quality of reflections, fog, smoke and lighting. It allows calculations to a per-pixel level. Not only does this make environmental effects look better, it make player interactions with things like smoke/particles easier to model.