I would go with vBulletin or SMF. Maybe phpBB when 3.0 comes out, has it come out yet? I don't keep track
vBulletin is by far the best and it's also by far the best when using it. I much prefer to use vBulletin forums rather than others when it comes to using them.
Yeah, pretty sure PHP 3.0 is available. Anyway, I've used most of the main forum software out there and vBulletin is the best for customisation in my opinion but phpBB and SMF are a good, free alternative but whatever you go with, just make sure it can import into VB later on, in case you do decide to upgrade. As for promotion, apart from common website techniques, you can try things like hiding premium content from unregistered users and forums requiring a minimum number of posts to access, to encourage registrations, activity & return visits.
i wrote something about this you may find interesting and helpful hopefully. http://www.forumseo.net/?p=4 and the follow up to it http://www.forumseo.net/?p=10