I am currently using a 6950 it has little head room for overclocking. Your best bet is the newer 22nm 7850 for power efficiency. Your CPU at 2.8 will bottleneck the card but only by a small amount.
an aftermarket cooler is a must with the x4 phenom I had one in my last build and it got hot. Also your motherboard plays a huge role in overclocking do to VRM and capacitor/mosfet count. Best tools for online AMD overclocking is AMD overdrive or using your gpu interface amd/ATI has an overclocking you and CPU function built into the suite.