Single disk solution for running many forums with mysql as backend db would render all forums very slow after a while. I guess roughly 30MB per forum installation files with preconfigured avatars, etc. For the database you would only need a couple of MB per forum, but as every forum grows you could be stuck with several hundreds of MB for each forum in the db. db backup on a single disk system would also slow down the forums. You should really try to figure out what limitations you're gonna have per user account/forum and it would be easier to make an estimate on how much you need.
If you already have thought of a forum product, then go to their support forum and ask any old-timer in there regarding resource utilization.