Like DarkSable mentions, it all depends on how the rest of your system is setup. A quality 500w PSU (which it seems you might have) should be able to power at least a GTX 770/780, or possibly a R9 280x. If you have an OC'd FX 8350 GPU and a lot of other things on your system, than you might want to stay around the GTX 770 or R9 280x.
What is the make and model of your PSU, so we can help you further?
Also what is the rest of your parts that your using in your build?