Three cards I can recommend from the least powerful one to the most. HD5670, HD5750 and HD5770. All of them are a massive upgrade from your current GPU. The question is what is your power supply since the more powerful a card you put in there the bigger PSU you need.
Teh 5670 is kind of meh.
It's slower than the 4850, So if you're PSU is 400W or more, get it. ( The 4850/GTS 250)
And 5750 is just to close to 5770 in price too be worth it.