I really don't know, as i said it depends also on the airflow of your case, ambient temperature, whether you keep your case open or not etc.
That card has already been overclocked (it says OC version right?). If i were you i wouldn't even try to overclock it unless i know what i'm doing.
Just a recommendation for the future, get GDDR5 instead of GDDR3. It may have more vram but GDDR5 memory is faster.