Thanks, Aristotle! Your comments on ikonboard are really useful. I'm trying to study the code to understand what exactly certain pieces of code are doing but have difficulty understanding because of DB and SQL, which I know little about. However, I do understand the obvious ones. I thus agree with you that the ikonboard engine seems to be a good bet.
I would like to try and understand YaBB because I find it easier to understand when textfiles are used instead of SQL database. I want to be able to "hack" the code a little here and there to change certain things if necessary. With ikonboard, I'm quite helpless but I may be able to do something with YaBB.