depends on your budget, get a 7770 if you are low on budget. or get a 660Ti if you can. 7950 maybe?
also, since you have a prebuilt, check the PSU, you might need to get a better one if you want a powerful card. a good 550w (seasonic, antec, corsair, etc) is a good place to start.
for reference i have an x760 seasonic, (it is higher than what i need i know) and im using gtx 670 with 4ghz procie