Daryl, in theory it's possible to have both an Intel and PPC boot from the same drive - see:
http://www.macosxhints.com/article.php?sto...006061610374449Note - read the comments - there are some corrections. And I'm also not entirely sure this will work on a recent Mini.
I don't think you should have backups for all three computers on ONE drive. What happens if it dies?
What are you using the G4 for anyway with the two newer machines? Just curious - since you're running 10.5.8 on it, it's not like you're running old software... (like my hubby who is stuck at 10.3.9 because he still uses Framemaker - and being stuck at 10.3.9 is now causing some real headaches!!!)