You have a PCIe x 16 Slot in that righttp://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=en&cc=us&docname=c02207223&product=4239296
So any good PCIe X16 GPU will do as long as the Athlon xII 220 doesn't bottleneck it. It's not a gaming processor.
Plus the RAM....
Go in for a upgrade if you want to be seriously gaming. For other gaming this rig is fine.
With a low profile HD GPU will suffice.