You will be able to achieve a very mild over clock but you won't be able to get very far with the stock cooler before heat creeps in, it's a pretty terrible cooler for overclocking. Increase the multiplier incrementally and watch the temperatures on the cores. Staying under 80c is what I'd aim at.
If you want to over clock you'll have to invest in a better cooler.
Get a hyper evo 212+ Im at 4.5 with temps under 73c under full load