Hard to say for sure as every card is slightly different.
I advice that you take the normal safe method and overclock it by small incriments such as 5mhz and give it a good test with something demanding each time. Look for any artifacts or degredation in image... keep going until you see them, and then drop 10mhz, and thats the safe value you can overclock too !
Tim
I am Homer of Borg ! Prepare to be... MMMhhhhh Doughnuts