Since IMacs have one FW800 port, this is what I do
G drive 7200rpm with fw800 and fw400 in/out
Imac to drive FW800, FW800 to second Drive, FW400 out of second drive to interface with FW400. One projects drive and a backup drive.
Most will recommend ext drive to stream audio projects to it. Keeps internal from working as hard and does not fill it up.