I wouldn't use such an electrical power consuming beast for a firewall. IMHO an ecological disaster
Why don't you just get an old Pentium I or II with 2 network cards, or a small machine like a Soekris or Via ITX.
I think running a virtual machine as a firewall is a bad idea. Programmers already have a hard time writing secure operating systems. You think that in their initial attempt in writing secure virtualization layers they suddenly do better? I don't