I've gotten very frustrated with the forum I have in the main site, so I've installed a stand alone forum. I could have it display within the main site, but that would just slow it down, so when you go to the forum it will look like a new site. I have not yet themed it, so it won't match the main site right away.
The good news is that I integrated the login module, so as long as you are logged in to the main site, you are automatically logged into the forums. There are a lot of setting I have to go through, so many features might be disabled at first.
You will notice new blocks on the main site (only if you are logged in), these will show any private messages you may have from the forum, new posts and topics, and who is online.
The old forums will not be ported over since there weren't that many posts.