If it were done correctly, and well maintained, it would be nice to be able to just go to a forum for a specific component (drive train, suspension, axles, whatever) and search/post appropriately. I think the number would need to be kept low to reduce confusion, and there are still going to be overlaps where your not sure where to post (especially for newbies). It really seems like it would be more complexity and more stuff to keep up with than it would be worth. I don't know.
I personally think a simpler setup similar to what they do over on PBB would be good. Maybe you could just make a "4x4 chit-chat" forum? Then keep the existing boards as "tech" only boards. IMHO, the real problem here is that anyone who has anything even vaguely 4x4 related will post in the tech section rather than the forum called "non-4x4". Their excuse (when given) is always along the lines of "well I mentioned a 4x4". Things like "How old were you", and "Look at this new ??? prototype", or "Look at what I saw on Ebay" (ridiculous posers generally), would be forced into 4x4 chit-chat by the moderators. Don't get me wrong, I don't have anything against the people who want to talk about that stuff but, seeing those posts accumulating 4+ pages of replies and always staying on top, bumping off requests for help, info, etc. gets a little old. This would give the people who want to participate in such drivel, er, I mean "discussion", a place to go (other than the place I often feel like telling some of them to go). {Hehe, sorry, I guess it's "one of THOSE days" /forums/images/icons/laugh.gif} Generally I just ignore them and refuse to contribute to them staying on top but, it gets old seeing them again and again. Anyway, offenders could be sent a message by the moderator and their posts moved to the chit-chat board. Repeat offenders (apparently relying on the moderator to post appropriately) could be simply deleted. Typing it in again or loosing it should convince even the most stubborn. Just make it a policy that, unless it is CLEARLY tech, it goes in chit-chat. I think that (at least where I would draw the line) would reduce the number of posts on the main board by 30%+.
Maybe I'm just a hard ass but, unless something better is suggested, that's what I would do. That way there is only one "tech board", and the maintenance overhead is much lower.