After playing D3 and multiboxing for quite a while, I noticed the game doesn't care or use much GPU, but it EATS CPU. If u are going to upgrade just to improve your D3 experience, then invest in a really good CPU + motherboard, and even overclock it. U will notice a big improvement in performance compared to getting a kickass GPU with the same i5 2500k. U can test this really easy. Overclock your GPU and see if there are any noticeable performance improvements. Then do the same with the CPU. Chances are, u will notice a big differece with CPU overclocking.

I overclocked my i7 4770k and got a nice 15-20% FPS increase.