That's a dual core, which is the big limiting factor. You need to stick to playing dual core friendly games, not games like GTA V.
To answer your question, of used AMD cards an HD 7790, HD 7850 or 7870 would be worth a look. There are newer cards based on those, like AMD R series 260/360, 270/370. that you could also use. Of new cards, the RX 550 and RX 460 are as much as I'd suggest for that computer.
If you want an older AMD but your power supply can't handle something like a 7870, look at HD 7770 or 7750s, those are lower in power demands, but not as fast of course.