You might want to look at DigiChat -
http://www.digichat.com. It is JAVA and is what we currently use. Very robust and scaleable the only users we have found who can't use it are WebTV users. The cost is not that bad (they have different options) and you can test it for free. Runs on Linux, Windows and I think the mac. Also they will do chat hosting if needed. The security is much better because ours is a "members only" chat and does not use IRQ so only our members can connect to it.
Just my two cents.
