Shoutboxes are extremely heavy bandwidth users. Even if you do have one, you have to ensure it is very well written because a problem with the code can cause it to suck even more bandwidth. A person I know runs a site that normally uses 1.5TB of bandwidth a month. An issue with the shoutbox made that 30TB last month. He killed it very quickly when he saw the stats
I think a better option would be an IRC channel, but then you need people to monitor it and that is much more difficult than monitoring a forum.