I'd suggest against it. There was a chat box added to another forum i'm on, and although at first it went great, it's like a ghost town now. That forum has a similar member amount to this one. We can't get enough people to post in the After Hours thread, so I don't see how the chat box is gonna be any better.
We use to have one because members demanded it. It didn't last long due to lack of use after it was up. Only problem with it was you couldn't see who was on. Tom had thrown a idea out there for a better chat room and no one offered to do anything.