I think its a not a good idea, never heard of chat box in forums. but still you can PM a member and then get his IM id and chat..
How about an IRC with chatrooms? On any other forum I would say that a chatbox would be a stupid idea, however I peronally think it would work well, with a per-board, or different room basis.
Get their MSN or AIM via PM and go on websites like meebo or beebo and talk. that way you don't have to download any programs.