5500? - Dont expect much(stable) headroom out of anything in that series.
EDIT: Well, what cooling solution do you have? If your stock, than you did alright. Last time I checked, there werent many if any volt mods or tweaks aside from raising your clocks. If you were to say, get a better
solution for that card, you would get farther, however how much, I really couldnt tell you.
Out of curiosity, how much did you pay for that card?