I think a Nvidia Gtx 760 would fit in that budget but depending on your PSU brand you may require a new one. Not much headroom even then. No overclocking, of course and regardless the CPU will become your bottleneck in games.
You could resell the cpu, videocard and psu and get a 550W PSU, FX 6300 or better and a GTX 760 or in my country ATI's 280 and 280x prices are dropping, might go with one of those.