For whatever reason, a self contained D6 forum module was never allowed to be included in the official modules but I've worked with this module quite a bit and I believe it is SERIOUSLY worth looking at, even if for just a basis or starting point or maybe even, if nothing else, a proof of concept. It can be found here: http://www.richardpeacock.com/blog/2010/10/rp-forum-drupal-6-module
It has most (if not all) the features all the "big" forums have such as:
Features (included without additional modules!)
Ability to ban users based on IP and IP ranges
You may filter out curse words (or any unwanted text) from posts
User signatures and avatars
User ranks based on # of posts
Moderators per forum
Subforums (1-level deep)
Automatic word wrap of long URLs or strings in the body of a post
View/Edit access control per forum (based on user role)
Custom search functionality"Report abuse" functionality
Administrators can move and split topics, as well as move individual posts
Includes an attractive default theme and icons
Simple WYSIWYG BBCode editor - may be disabled, and fckeditor or any other editor may be used instead
Smiley package (included with WYSIWYG editor)
Automatic RSS/Atom feed of posts (may be disabled)
Users can "subscribe" (be emailed of replies) to posts
Automatically interfaces with Privatemsg and User Panel
Comes with a configurable "Recent Posts" block
All posts are CCK nodes, not comments. So users can easily attach images or files or any other field type simply by adding the field to the rp_forum_post content type.
Spanish language translation provided by Maximiliano Méndez.
Italian language translation provided by Bruno Ravera.
So, I reiterate, I definitely think this would be worth looking at and possibly converting to D7, It's easy to use and configure so...again, not sure why it was not allowed into the D6 available modules but, I think it should have been.