Time Machine - Questions!

I'm thinking of upgrading the household computers to Leopard (mostly for Time Machine, SMB and networking improvements). I would dearly love to be able to have all the Macs backup to a central location. I already have a G4 Mini with a Drobo attached for external storage, which all the computers in the house are able to access over a 100mb wired connection (the wires are old, no GbE for me *tear*)

Here are my questions:

1) Can mutliple Macs use the same central computer for backup?

2) If this computer was running Leopard, would it be able to use itself for backup?

3) Is it necessary to get a new external disk exclusively for Time Machine, or can it "share" with the current data repository? We have about 300Gb of music and movies.

4) Would I be able to back this external disk up to the other "Time Machine" disk, if necessary? Assume I have enough space.

5) I read the thread "Time Machine over network... some thoughts" and it has confused me slightly. My understanding is that you can tell Time Machine to back up to any SMB mount (the G4 shares the data over SMB), but it's unreliable. What do I need to do to set up the "safer" AFP? Before today I'd never heard of it before.

6) Lastly, has anything been done about the Leopard firewall issue?

Thank you everyone in advance!