Well since AMD was recommended, i just gotta throw in my 2 cents, a 7750 might just be perfect for what you got, and shouldn't bottle neck you too bad. Now if it was me i would wait for NVIDIA's new kepler line for CUDA. most games your playing love some CUDA. heck thats what they're graphics engine is running from. NVIDIA's technology that is
I'm not sure what CUDA is, but, I am happy going for older GEN cards.