I think it depends on several factors on what you can OC your i7 to on a stock cooler. Depending if you get a C0 or D0 stepping i7 the D0 seems to OC more and at lower voltages. I think on the stock cooler you will get anywhere from 3-3.6 GHz depending on the stepping and whether you get a magical CPU or not. When I say magical, I mean some people seem to get CPU's that OC at stock voltages to like 3.8 or 4.0 GHz, but this is VERY rare and not the norm.
My thought is, if you are intending do a heavy OC on your CPU, get the cooler right off the bat. Reason being, all of the higher end coolers require a backplate. This means you would have to gut your PC and remove the motherboard in order to install it. It would be a BIG time and headache saver to install this at the beginning, TRUST ME. If you install it after the fact, you have to worry about moving wires around and disconnecting everything and putting it all back. It gets frustrating.