We get a lot of live video questions regarding church use, also. I have absolutely no experience in live video, but this month's eventdv magazine covers several ways to do this in their "the amen corner" article on page 24 (May 2007 issue). It might be worth checking out. Of course, you'll want to make sure that whatever device you get also supports green screening.
Also - I think Serious Magic has a way to do this with a single camera firewired to your PC.