Although it's possible to connect computers via FireWire, it's more efficient to use ethernet. From what I understand, firewire switches are not the world's fastest products; most of the ones I've seen have one or two 400mbps interfaces, and several 100mbps ports. (That said, I've never seen a FireWire switch that big before, either.)
If I were you, I'd just get a fileserver or two, and give them multiple FireWire cards, so as to keep the FireWire interface from being the bottleneck. Alternately, buy an old-ish PC(~1Ghz Athlon, 512mb RAM, 4 IDE channels, and 5 PCI slots), a gigabit ethernet PCI card, and four PCI IDE cards, which retail for about 13$ at geeks.com. Then, buy 12 400GB drives, and wire 'em all up. The PCI bus would slow you down a bit, but it would be a decent way to back things up. And gigabit ethernet is quite fast.