KDX require that u register and pay...

KDX has and is always a free download. They ask for you to pay for the certificate to be able to use the Address book and for it to keep addressess.
And no, you dont need a server for KDX seeing as it is a client to client program. There is a KDX server version but all that is, is some scripts that auto connect you to everyone else thats on the server.
And yes, Ventrillo and Teamspeak are being ported to mac and even Ventrillo has a Mac os X version of there server but the client version have been in production for years now and dont expect it anytime soon. Read the Vent Mac Forums.
voo