So I pulled the HD from a Dell, stuck it in a MacPro -- and booted it up.
Ain't Linux grand ? 
I'm waiting on software and a HD before I can backup and 'hackgrade' the Mac Pro to OSX 10.8 (which isn't officially supported by Apple on older MacPros). AND I've got a Dell 960 gathering dust due to a bad PSU. Don't know why it took so long to occur to me that the Dell HD was also a SATA, and had Linux Mint installed. So at least this eight-core processor isn't sitting idle the entire time I'm waiting for parts ! I even got the NVidia driver for my GPU card installed and running, so I can run bigger jobs than were possible on the Dell (NVidia driver compatibility was part of my reason for wanting 10.8, so that's kind of ironic).
Who would have thought that one day you could swap CPUs and even OSs between Dells and Macs ?