FreeNas is based on FreeBSD, hence requires a x86 machine.
Rather than using a G4 or G5 I highly recommend the FreeNas route.
Although Mac OS can deliver most services FreeNas can, the really tempting ones like iSCSI or ZFS are not supported though.
I recently build a new FreeNas machine for my home network based on ZFS. These are the parts I bought:
9 x 2TB WD 20EADS (8 for the server, 1 spare)
Intel Gigabit CT Desktop PCIe NIC
4GB Kingston RAM
Asus P7P55D Deluxe (9 SATA ports, 8 for the storage array, 1 for the system drive)
Intel Core I3 530 (smallest I could get for the board, plenty of fast for the FreeNas box)
530W Be Quiet Power Supply
and a case with 9 5.25" bays for 3 hot swap hard drive bays that each hold 4 drives
Some silent fans and you're done.
Total cost was about 1500€. The drives are configured as a raidz2, so two parity drives, which gives me a total capacity of 12TB (advertised of course).
I haven't measured the power consumption of the machine yet, but I reckon it's below 150W.