Your power supply will play a huge part in your selection. If you have the power supply for it, I'd suggest looking for an HD 7950 or 7970. Since you are willing to shop for used cards, these would be the most performance for the lowest cost. The downside is they use much more power, so that's why your power supply is so important.
What you have now is so outdated that even an upgrade to an RX 460 or GTX 1050 will be a big and worthwhile upgrade, if you need a low power demanding card.