Whats the cheapest way to host a forum with about 400 frequent users?

[wozencl]

Those two would only matter if:
1. The media is being hosted on the forums themselves. If stuff is linked to offsite, it doesn't count at all.
2. The media will bring in a mass of additional users.

Of course. I've never had to run a forum but the ones I'm familar with, as a user, are relating to unreleased/cancelled games - you can imagine how much interest they get.

Again, limiting the download of hosted content to registered users would be a good idea if you're going to be attracting a lot of guests. Otherwise chuck the files on a free host like Mega****** *shudders*