I would go with 4 internal one for the system and three set up as a raid 0
the SATA raid will knock the socks off of any external FW drive
as far as size, that all depends on how nuch storage you want and can afford 4 500’s would be awsome.
One thing to knote is that you will need to take advantage of the FCP crossgrade soon so that you will have a version of FCP that will run on the MacPro (the intel processor needs the universal verson of FCP)