One system will handle it. Record on the camera and off-load it to the computer afterwards, record the audio directly to the computer. You could use a separate system, but what would be the point unless you just wanted to have it? You can use either a cheap graphics card for dual outputs or a splitter (GFX card would be cheaper).
I see a lot of churches that want to use a laptop, but that's a big mistake as far as I can tell. A desktop should be used in the sound/media room. If the pastor wants to use a laptop, let him; give him a few flash drives to use to transfer his PowerPoint on.
A decent pre-built would serve your church, or you can build one. Try this one:
The Dell Online Store: Build Your System
Bump it up to Vista Home Premium, 4 gigs of RAM, add the 3450 and the firewire options. Drop the pre-installed anti-virus crap. Ask for someone in the congregation to donate MS Office (most versions have multiple licenses with them) and install your own AV. It comes with a 19" LCD and will still have another output for the projector. Install some codecs in it and you could show DVD movies.
Since it has a C2D in it, it will be very able to handle your video needs as well. You would be well served to invest in a good sized external drive to store the media on in addition to burning it off. it comes with a 250 gig hard drive, but Dell's prices are too high to justify upgrading it through them. The only reason I suggested bumping the RAM when you order is because they will install 2x2gigs.