We are using 3 SATA2 250 Mb each, inside the trays of the new MacPro, that gives me something arround 700 Gb. and each one is set as SATA2 3Gb/s.
I set up those as a RAID 0 config, using the MAC OS X RAID Software, bult in on the Apple OS system. We have other 2 MACS using MAC OS X RAID Soft, and works great.
You are correct, we also have the fourth drive ( another 250 Mb SATA) with the system, MAC OS X, Catalyst etc... and the RAID config is just for Content.
Gian