you should open a forum for a new trend / game / movie - then you have a chance. It will be tough to start a forum in a crowded market.
Depends what you have on offer, if you are going to do something unique with lots of useful stuff and posts in like on DP go for it, otherwise there is no point! DP is making $5K a month from what I heard time ago, probably a lot more now! That is not bad at all.
not bad at all indeed it seems like you can boil it down to traffic. if you can pull traffic in, then you can find success as long as you can manage it.
Forums are very informative actually. You can actually ask anything you wanted and surely somebody will answer your question. And not just 1 answer people are like advicing you here.
There are actually some fantastic tutorials out now to adding advertisements to forums, especially for VBulletin there is a great Tutorial for adding Advertisements and it seems to work wanders! I have just started a VBulletin board, that is where I found the tutorial after some searching http://www.vbulletin.org/forum/showthread.php?t=107965!