Moving forum out of the main database I have a Drupal 6 site with an increasingly bloated forum. I am wondering whether it is possible I can move the forum out of the main site's database to a new database, possibly hosted in a new server. 

So  I guess my question is a special case of distributing a single Drupal database over several servers.
One option is to have a multidomain setup where forum.yourdomain is your forum installation and www.yourdomain is your main portal. Then you could have a single sign on for both domains.