If you "max out the card" using the stock cooler, I think you're going to end up with an expensive paperweight and not simply a bluescreen. As has been mentioned above Overclocking carries risk, and you are better off simply leaving the card alone if you don't want the risk. However Ravi has made an excellent suggestion which looks more like the midpoint for this card. If you overclock, that is probably the best destination given the configuration you have.
More appropriately however, Deuce65 said that you should overclock it to whatever happens to be stable. Since you can push a card too far, you can find this out by making incremental increases, and not simply by pushing everything to maximum in one go.
Additionally, please do select the appropriate response (ie not mine) I think your question has been appropriately answered by the above.