hi quizzy,
I think max 5 has net region render.
and the radiosity is calculated by each of the computers in the network if solution isn't calculated before sending to the network. if you are doing a brute force GI you can use network rendering also. you should chekc this before getting that xeons...
Tom's Hardware
http://www6.tomshardware.com/cpu/02q1/0203131/
Athlon MP vs Xeon
Anandtech
http://www.anandtech.com/IT/showdoc.html?i=1606&p=2
Athlon MP vs Xeon