[Kaelon]

With 20-40 users online at a time, I agree with Reeve of Shinra -- vBulletin 3 should run fine with you. However, you are right on the cusp of what a virtual server provider would consider "excessive CPU usage" (with around 30 people, CPU usage begins to reach monopolistic levels on most server configurations), so you may be asked to go to a dedicated server if you have not already. That said, the same situation would arise if you were using vBulletin 2.

vBulletin 2's superior resource management (largely because it has fewer features) of memory is more apparent on extremely large servers, but vBulletin 3 manages with better queries.