No simple reply to this one! Different CPU's give off more heat when overclocked by the same amount, so some which don't give off so much heat will be able to go that little further on a stock heatsink.
Having said that, overclocking always results in an increase in temperature so a specialised heatsink will always be advantageous to your overclocking attempt.
It also depends on what stock heatsink you have, some are poor... really, really poor; but some are actually ok, so again it's another variable.
Overall, It's always going to be a better idea to upgrade the heatsink and fan if you're going for an o/c. Depending on which model you get, what fan you have on it, what case fans you have and the ambient temerature in the room the computer is in, you should get a fair amount of an increase in performance (ie DEcrease in temperature at stock speed). I can't really give you figures, that would require a comparison of all common standard heatsinks verses all specialised heatsinks out there (maybe this exists, google is your friend) but you should lose anywhere in between 5 and 15C as a total rough ballapark estimate.
In conclusion: if you're going to overclock, buy the CPU by itself and then ask the chaps in Anandtech what heatsinks they think are decent. Pick up one that's well recommended and you'll be laughing. One final point: these days people are very much into their quiet computers, so it's yet another thing to think of when choosing.
Good luck.
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