There are a few Linux distros for PPC Macs. Ubuntu and Gentoo work well on my Mac Mini. And there is FreeBSD (which FreeNAS is based on), but I haven't tried that on a Mac. You'll have to do a little configuring to get them to act like FreeNAS.
To answer your original question, I believe that you can only boot PPC Macs from a Firewire drive (that's easy), not a USB one, but you can boot Linux or FreeBSD from the hard disk
Ok so you mean that I cannot boot from an external USB but it has to be a firewire USB/Hard drive?
About the boot Linux or FreeBSD from the hard disk: so I could format completely the hard drive and install FreeBSD or Linux, but PPC distros, on it?
(I know thats possible with PCs but Apple has restrictions on its Macs so thats why I'm asking...)
Thanks for the advice.