I decided to go with an interim solution - I'd like to get a couple of more years service out of my Mac Pro 2.1 (early 2008 with 2 quad core 3 Ghz Xeon processors). This will give me a chance to assess the next generation and to avoid buying v 1.0 of the next generation.
So . . . . I ordered some hardware from OWC:
32 GB of ram - the last time I looked 16 G was the largest available for this machine, so this was a very welcome surprise.
Two 480 GB Mercury Extreme SST drives configured as a striped RAID. I used a kit from OWC to install these in the second optical bay. This is not for the faint of heart because it required a fairly extensive disassembly of the machine to expose the attachment point for the cables. I'm using this as a boot drive. Carbon Copy mirrored my old boot drive to it without a hitch. I configured this with a separate RAID 0 partition (total of 64 GB) to use as a scratch disk (DigiLloyd has instructions on how to do this).
Two pairs of 2 TB 7200 rpm drives in the drive bays, set up as striped RAIDs.
I had an e-sata card so I put the old boot drive and one other in an OWC enclosure and connected via esata.
I still have the Drobo - 8 bays filled with 2 TB drives, for a total of 10.8 TB available after overhead for two drive failure redundancy. For some reason iSata (as opposed to e-Sata - what bozo decides on the names for these things?) doesn't work with my Mac Pro so I'm limited to a Firewire 800 connection.
I'm using the SSD Raid as a boot drive; the SSD partition as scratch; one pair of RAID 0 drives (4 TB) for working files; one pair of Raid 0 drives for Lightroom indexes and support (the large drive lets me save full sized previews when I import without an concern over how much space they are taking up); I back up LR to a drive in the OWC box; the second drive in the OWC box has a mirrored boot drive which should make recovery relatively painless if this old beast ever fails; I use the Drobo for older working files and backup.
Overall the machine feels very responsive and fast, but not blindingly fast. With IQ 180 files Editing controls in C1 and LR feel natural with no delays or jerkiness. In C1 100% screen refreshes require a quarter of a second or so to reach full resolution. Files process in about 11 seconds. In LR when I move to a new image the "loading" tag shows for about 4 seconds; after that zooms and scrolls are instantaneous.
A two frame stitch in PS, starting from LR (again using IQ 180 files) takes about a minute and a half; flattening the layers only a second or two. A four frame stitch took 3 minutes. Saves for these large files are dramatically fast - the four frame stitch is 11,000 x 17,800 or 195 megs, the file is 1.1 GB; it stored from PS in roughly 2 seconds.
All in I spent a bit over $2500 (I managed to recycle Sata drives) and 5 hours or hard labor - for that I bought a couple of years of time.
Thanks to everyone who responded on this thread (and thanks to those who participated in
Mac Solutions for Medium Format) for your very helpful advice.