Most definitely two databases if you're doing a large scale project. At least there will be a separate db if someone is perusing the forums vs looking at the site. Otherwise you're going to have a db that's trying to serve everyone at once and it's going to get bogged down quick :<
It'll cut down on the amount of traffic that will be coming through if you use separate. My two cents at least.
From a performance perspective, do you think it's better to do this one of two ways:
1) One database that houses all of the site content as well as forum content
2) One database that houses all of the user content as well as the forum content, while there's a separate database housing the site content
The two are going to be integrated into each other, with the users from both being the same for either side, I just can't decide if it's better to have a massive, slightly cluttered database or two smaller ones, but with two separate database connections at certain times.