there is no specific limit to how much a certain card can OC

it is all exprience because each card is different as well as the whole system as well

you limit reach is due to several factors

1. heat - get a better hsf to lower the temp
2. psu - oc - ing does put a little strain on the whole system, not enuff juice can cause system down.
3. try voltmodding - this will improve the max OC-ing , but require good PSUs