Another "why not?" purchase - a Craigslist special. Listed as not being able to boot, no specs, new 1.5 TB HD, and 2 20" aluminum cinema displays. Also came with a wireless and solar powered Logitech keyboard and a wireless Apple mouse. Bought the set for $50.
Turns out to be a 3,1 - and it did boot. Shockingly - because the interior was caked with dust. A good vacuuming and it now runs perfectly. Something to tinker with!
It's a fairly basic 3,1 - 2.8 GHz 8 core, with 4 GB RAM, the original 320 GB HD, and a Radeon 2600 (which I've replaced with a GT120 I had). Ordered 32 GB of RAM (for $35!), and will also order an Airport card for it. Otherwise, it's done - might give it to my son if he needs a desktop. And both of the displays work (and came with the power supplies).
A LONG way from the 2013 MP I've been playing with, but should be fun to tinker with.