If you have tried Sharepoint discussion boards, you already know that you cannot offer a PROPER forum experience with it. It's still very primitive and it will take considerable time and effort you to extend and customize.

Fortunately I found that there are some third party solutions, and Lightning Storm Forum is a very nice alternative. I'ts not free but not that expensive either.

You can try it for free and can have a proper forum running in your website withing less than an hour :)