....nothing in the AMD or Nvidia camps below the $100 mark will give you what you might expect from a 4 generation upgrade, though AMD does have the Radeon R7 250 which is a mild upgrade starting at about $80 and seems to be about on par with the GTX-650 in performance, while using considerably less power than the GTS 250.
What he said. Until you have a couple of hundred to spend of a GTX760 or whatever the next performance per dollar bargain is, stick with the GTS250. Anything you are looking at now is either slower or not worth the money in performance gain. And I agree with you your GPU is currently the bottleneck on your PC.