I agree that monetizing a forum is a different feat than simply setting one up. I think that good forums grow organically because of good discussions. I think that it's a mistake to monetize too early. I've seen that happen before and those forums are not all that exciting. Your best bet is to find a forum that can grow because of the information that is shared, and only after steady growth add some money generating element to it.
I mentioned about this in this forum, but I mention it again here. It's pretty tricky. As you suggest, you don't want to place too many ads at the beginning. But if you do not do that, then it gets pretty difficult to place ads later. It can turn off existing members; it can make existing members pretty unhappy. You don't want to place ads too late. You want to place ads in the process of developing your forum to set a certain expectation.