CM,
My understanding from the US RME rep is that it is necessary to use the hardware i/o for the loopbacks. In other words, you can not use 64 channels for software loopback and 64 channels for audio i/o simultaneously. Each software loopback track eats up a hardware track.
Also, I haven't yet been able to find via Google any 15K 300GB SAS drives on the web. Although Apple is already offering them at $800 each. Apple's SAS RAID card is $800 also and is required if you are using the SAS drives. So that would be $3200 for a 900GB RAID. A little steep I think - especially considering the cost of the Mac Pro, monitors, 16-32GB of RAM, etc. Using an UltraSCSI, three 10K drives and an enclosure would be about half that price.
For my setup, I'll need more than the two PCIe 1.0 and one remaining PCIe 2.0 slot. So an expansion chassis is unavoidable. Unless I stick with only one Mac Pro. Then I would have only one MADI card, one SCSI card and an extra FW card for the audio interface to the G5 reverb machine.