I see 123flashchat is leading the poll but no one has actually mentioned it within the thread, can anyone shed some light on it, I'm on there page however if it were my product I would obviously tell everyone its great, I would like to hear from fellow vBulletin admin how intensive it is on the server, as I currently use flashchat which knocks the load up quite a bit.
All the server side solutions including flash will greatly increase your server load once more than a few people join the room. How many you could handle depends on a lot of factors, but in general I wouldn't count on a shared server to support more than 5 or so during busy times.
I recently added AddonChat free. It works, and zero server load since they host the chat on their servers, but the free version is sorely lacking in features.