Any gpu that is as good as an r7 250 or below will do very nicely, even with some games at up to 1080p resolution. And there are great prices for those entry-level gpu cards. Just make sure that your PSU is up to the task first (which probably is already...), and that your motherboard has a good working PCIE slot available.
My advice ?
Start saving money and plan for a whole new budget desktop PC the first chance that you get, preferably one with a good AMD APU that will replace that good 'ol Sempron.