I have installed four 750GB SATA2 HDs in my brand new MacPro. Then I have added one 250GB SATA2 HD to one of the 2 extra SATA2 ports available on the motherboard. I had to cut a piece of the front fan to make it possible to install the cable. I am going to look for another SATA2 HD in order to add a 6th HD. To power the 5th and the forthcoming 6th HD, I have been using the power line available from the optical drives.
I can not wait to get my Decklink HD card!
Hereafter are benchmarks performed with the Disk Speed Test utility provided with the Decklink. I have been running it 15 times with different RAID 0 settings (32k, 64k, 128k, 256k block size), and variation in read/write was not higher than 2 MB/s; so it is really stable and reliable. In a near future I will probably try to setup an internal RAID 0 based on 6x 750 GB SATA2 + un ATA HD for booting. MacPro are great!
