If you haven’t made your purchase, you may also want to look at XenForo (www.xenforo.com). It was designed by two former vBulletin developers, and is a speedy, lightweight system with lots of features and good integration with Facebook and Twitter. There’s a growing add-on community, and good support from users and direct from the company.
All my forums are moved to XenForo, if you want to try them out for size. We ended up selling off our vBulletin licenses. As to IP Board, I played with the demo but never really liked it all that much. You may also want to consider MyBB, the board DreamHost uses. It doesn’t have all the features of the commercial brands, but it is free and not a big drain on system resources.
Personally, I’d worry over some board systems handling 50-150 consistent users on a shared system.