No separate spam gateway -- that's gonna hurt. When we took the mail server offline to switch to an outsourced solution (that isn't working out very well) our primary domain was processing nearly 500,000 messages daily, only about 30,000 of those were non-spam.
I've cleared a server for the initial install. Should be interesting since it needs to use a name/setup that's already running on another server.
I'll try to document my steps as I go along so this can become a howto thread.