I just followed the advice on here - http://techgage.com/article/overclocking_intels_core_i5-750_i7-870/3
"The process of overclocking the i7-870 was pretty much the same as the i5-750. We first tried to achieve a high overclock using stock voltages, and thanks to the slightly higher multiplier here, we were able to get by with using a lower bus speed (173MHz vs. 155MHz). Not only that, but we managed to achieve a slightly higher overclock, as well... 3.72GHz!"
I only have a stock cooler so I'm a little cautious of following the next steps about pushing towards 4ghz, but hopefully I can get a new cooler.