There's really no way to determine this, and at this price range, it doesn't matter, because the bottleneck isn't going to be the CPU.I assume you first figure out how many draw calls the CPU can do. And then find the cheapest card that can handle at least that many.
Only 3GB of RAM is going to be somewhat limiting on a gaming machine.
What motherboard and power supply do you have?
I was going to say a 570 or 660.