The drives in the Intel Xserve are mix and match, SATA and SCSI. It has a two-for-one SATA/SAS controller bolted to the Intel I/O controller. Seems fair to assume the RAID will also do both.
Regarding 750GB ADMs, Apple's official line is that they haven't approved (they tend to use the word rated actually) the 750s for use in the current RAID.
I have always thought it makes more sense to buy the smallest possible drive modules from Apple, then swap the disks with your own choice. Apple claims to test their ADMs thoroughly, and this should mean they are more reliable, but they still fail, so I reckon buying a stack of off the shelf Seagate drives is a better bet. That way you get a 5 year warranty, and its still probably cheaper than buying big ADMs from Apple. Plus Apple drives are odd sizes, so you have to get a replacement from them, even when the warranty runs out in order to rebuild your array.
For example, Apple will sell you a 7TB (14x500GB) for $13000.
If you instead buy the 1TB (4x250GB) and 10 extra 250GB ADMs, then get 16 750GB Seagate drives at $300 each, it totals less than $16000. You have 2 spare drives, and 5 year warranties on all 10.5TB.