Advice on Graphics Cards

[Solace]

6GB of RAM are good only with a triple channel architecture (that depends on the chip). 4 GB are actually plenty for most gaming needs, and if your chip supports double channel, better than 6.

Still the cpu bottleneck is a valid point. That is going to cause a large drop in fps.
Not a hypothetical drop, a very real, large drop.
My system with that cpu, would loose about 4k points in the benchmark, on estimate.

Bottom line. The gpu is going to give you the most increase in fps, at the least overall cost.
You would need to entirely rebuild the system, to remove bottlenecking.
The mobo, cpu and memory would need to be replaced, to make as much difference as the gpu by it's self.

To the OP:

Get one of the GPU's recommended and call it a day, if you are not looking to spend a thousand dollars or more.