The answer requires a deep meditation. You have to think that if the site in question has heavy traffic, will be worth it, otherwise can cost us a great effort. In the latter case we can move forward, but we run the risk that the project in question fails and we end while, with the consequent loss of time. If this is the first case, a forum can attract us visitors muchismos to perform queries on a specific topic. Think you need a moderator or experts in the topic moderators, who are able to answer questions from users. Similarly there must give special mention to the inappropriate messages (SPAM), that there will be that eliminate them. If we keep them, the site will lose quality and the users finished marchandose. Maintaining the quality of the Forum, users will return and will be continuously updated, and new content, something that the search engines like them greatly.
Add, that the Forum should be friendly from the point of view of search engines, with urls promocion_de_paginas_webs.php type using mod_rewrite, rather than dynamic urls like pagina.php? id = 60. Similarly, the configuration is recommended to have the title sent in the message by the user, appear as page title. This part is equally important and must have everything ready before you start to run the application. This article to integrate a forum within our web?