Firewire400 = 400Mbits/second Duplex 4.5 Meter cable length limit.
Firewire800 = 800Mbits/second Duplex 4.5 Meter cable length limit.
Gigabit Ethernet 1000Mbit/second Duplex 100 Meter cable length limit
I think that 1000 is larger than 800 or 400.
Also the cables for Ethernet are dirt cheap, firewire is not. Though there is Firewire support for networking in a number of OSs it is generaly through Hubs. Can you say collision? There is a standard for Firewire over Cat5 cable to 100 Meters but I would love to see the cost of the hardware for that one.
Gigabit cards can be had for around $20.00 or so and a 5 port Gigabit switch for around $35.00. The cables are a buck or two.
Go Gigabit!
Mike