Also, assuming your monitor is 1920x1080 and maxes out at 60-75Hz refresh, it might well be worth considering an RX 580 8GB instead.
It'll definitely perform better than your existing card, but not be overkill like a Vega 64 would be.
EDIT: I've been variously seeing great deals pop up (but get sold out quickly) on RX 570 and RX 580 cards, both 4 and 8GB variants. Depending on the sale, it might be worth going with the 570 vs the 580, and/or the 4GB vs 8GB.
Now, if your monitor's refresh and/or resolution is higher than what we've been assuming, then that changes the equation.