Well, you can. However, you need to figure out if this overclock is stable. The heat is not the only thing to consider when overclocking.
What you want to do is download 3dMark 11 and run the free version in performance mode. If the card can get through the entire demo without errors, then its safe to say that overclock is stable. If it cant, then you have to go push the sliders down and keep testing until you get it stable.
One thing to note: Go ahead and give the card the extra 20% power boost. Slide it all the way to the 20% for any overclock, that will make it more stable as well.