I just wanted to mention that there are also very good SaaS solutions out there, that allow you to setup a Q&A forum in just a couple minutes (such as Answerbase.com and GetSponge.com, although I probably recommend Answerbase given their credibility and the clients they serve) and that will allow you to not have to worry about security updates, new features updates, etc since SaaS allows the service to take care of all that for you.
Since traditional forums are pretty much a glorified directory structure, a lot of people consider these Q&A systems the next generation of forum software. Forums can get a bit toxic after awhile, since you have members that just shoot the breeze back and forth so the content isn't extremely valuable and it takes forever to sift through to the good stuff. The Q&A format provides tools to identify the good answers to questions and keep them high on the page right under the question, so your other visitors/users have a positive experience.
I would check out Answerbase.com first since they have a few good brand names under their belt, and they have pricing ranges for pretty much any size site.