carbon copy cloner is probably the best app for you to use.
The intel and ppc will both insist that the drive has to be formatted 'their way' to work, gpt for intel and aps for ppc, but in reality the intels can also secretly boot off aps, so you will need to partition it APS for it to work on both computers as a bootable volume.
ccc will only do the initial copy. if you want to keep them updated as live bootable backups, I'd recommend looking at RsyncX, which is a user-friendly graphical interface for rsync, that is capable of updating bootable backup volumes.
