um. what can your server handle?
Invision Board (IMO this one feels alot more clean to me, both from the admin side and the user side)
phpBB seems everyone has this one
actually, a good google search will turn up more message boards than you could humanly handle. lol. most of the major ones (invisionboard/phpbb) have relatively similar features, and feel about the same anyway, so it just comes down to which you want to use. I believe both come with installation scripts to make it even easier to get up and running...
btw: do people actually use message boards? if they're quiet initially, no one will ever use them.