GTX 460 1gb, HD 6850, whichever one you find a better deal on would be my recommendation. A step down from them and also good for it's price is the HD 5770/6770.
Personally I would go for the 460, my HTPC has one it plays LOTRO great, the DX 11 water is really cool looking.
I assume you have a Q6600 CPU, you should check out the guide pinned to the top of the overclocking forum here and try and overclock your CPU. People usually get 3.0 out of those chips easy on the stock cooler.