When the M715zone.com forums crashed, I set up a "temporary" home, that we ended up using for about 6 months, until we got the new Zone totally rebuilt.. We had over 500 members signed up, and around 25,000 posts in that time.
We used proboards.com, and it was a good experience all around.
Only downside is, your data is trapped there...only way to save any of it (other than exporting PM's) is to cut and paste -- they back it up, and can restore, but you have no such access to the database. Also, like many of the free forums (and some paid hosts, too) if you violate the terms of service (i.e. allowing nekkid posts, etc), they will delete your whole forum if it is not taken care of by mods immediately. Run a tight ship, it's not a problem.