Pelican Parts Technical BBS for the most knowledgeable people and the most mature (6speed gets too juvenile with a lot of newbies and baller types, imho.)
http://forums.pelicanparts.com/
And, as already mentioned: Rennlist. It's also more mature.
http://forums.rennlist.com/
There's also a forum started up by the original owner of Rennlist (Rennlist was sold to Internet Brands not too long ago.) It's called Reutterwerk. It's not super active, but it also has the same type of members like Pelican and Rennlist.
http://reutterwerk.com/forums/index.php
p.s., if you're shopping, then give these guys a call to help you find the car you want (if not buying new.)
http://www.truspeedmotorcars.com/