I'm planning on installing this for use on a corporate Intranet with 6,000+ users who will hop on this board the first day it's introduced.

I fear that it might bring my server (IIS) to its knees and I would like to avoid that at all costs. What's the min suggested hardware I should be using with this many users in mind?

Always difficult to predict this in advance. As a busy UBB requires a lot of disk I/O, you may want to start off with a fast SCSI disksystem.

Any decent PIII will do fine. You can start with 128 MB of RAM, and add more along the line if needed.