[MagnetiCat]

Go for an IRC solution. This is the only way to go if your forums are large or are going to be in the future. All other chat scripts can kill even the best servers with a relatively low activity.

IRC won't add any overhead to your server. The only problem is that you won't have user db integration in IRC of course, but the advantages really outweigh this limitation.

If you really want to have a chat box your users can use from their browsers without downloading mIRC, just find some java based IRC script - there are many great ones around with very small footprints. Try looking for "java IRC" on google.

You can also deploy one of those nice eggdrops inside the chatroom to store chatlogs, user information, and things like that that you can then display in your website.