Hi BW85
Factory OC'd cards are well tested and stable at the overclocks that are shipped with them. They also typically have better coolers installed on them. So you won't have to worry about factory speeds/temps if the card has been overclocked by the manufacturer. You can usually increase clock speeds above what they ship with as well.
However, the same is true as with any other GPU, you want to make sure you have good airflow through your case.