If the budget is around 400 USD and you are talking about directx 12, then you should go AMD (I'm thinking about the R9 390X). Why? Because AMD is more optimised( right now) for Directx 12, because their GCN architecture also focused on asynchronous compute and they've developed Mantle which, in turn helped develop Directx 12. Also, i think you should go for i5 4690k, or i5 6600k if you have the budget (but that will require new motherboar, DDR4 RAM etc.)
P.S. I suggested this without knowing other specs you have...