wow, the way i see it, is that is ONE VERY BIG WASTE FOR AN AWESOME COMPUTER. lol. bernied is right, if all that the computer is gonna be is a firewall, i myself would put my worse computer up front, and instead of having a file server, just share files across my network thru the router. if the file server, however, is meant to be accessed by outside ur network... i'd STILL put it behind the router, and activate port forwarding on the router, cause that thing can do way more than hold files. so, if you've got another computer with 2 ethernet ports, u should put linux on it, and set THAT one up as ur firewall.
now, about the firewall itself.. in order to get that setup, u'd have to do something like the Windows ICS (Internet Connection Sharing). as i am new to linux i dont know how to do this... but surely it's possible. someone else on these forums probly has the answer to that.. hehe, just not me [