Well the best upgrade for gaming is almost always a Gpu, and since you have a 5000 series I would strongly recommend a new gpu. Since you are only running a 500 watt psu I would say the Radeon 7750 is the best upgrade to price ratio. http://www.tigerdirect.com/applications/SearchTools/ite... This will also leave you with a 30 dollar headroom in your budget.
you need at least a hd 6870/ 6850 to really make the upgrade worth it but the dilemma is that your cpu will be a bottleneck so i think that maybe upgrading you cpu por a fx 6300 if you have a am3+ mobo and wait for the release of the hd 9000 series that certainly will bring the hs 7850 1gb prices down could be a better idea.
Update: the gtx 650 ti is a good upgrade also and is the best $135 card out there.