I've done a bit of research on this because I have an 5 year old CPU. Mine is the i5-650 which I just checked is slightly worse than the x4 750k.
What people have told me with my CPU, is that a GTX 750 ti/R9-270 is about the best card you can get, without the CPU limiting the GPU.
I decided to go with the HD 7790 and I not having any bottlenecking. So you should be safe with a mid level card.