Not exactly the answers I was hoping to hear but I do appreciate your answers.
The answer to the logic board upgrade question pretty much applies to just about any Mac model (Mac Pro, iMac, MacBook, MacBook Pro, MacBook Air, Mac-Mini, etc.).
There are some situations where there may be a very small possibility that a given computer model may be able to be upgraded/swapped with a newer models logic board...but these situations are very very rare. If they do exist...it's rarely done...with one main reason being cost...and another reason may be that it isn't worth it since the cpu & gpu performance increase isn't worth the effort. I totally understand (in this case) that it's not a performance issue...but a max. OS issue.
But in reality...Apple is always changing things...which makes logic board swaps impossible. As harryb mentioned...the Mac Pro 3,1 logic board that is needed to be able to run an OS newer than 10.7...needs a different power supply. And who knows...the Mac Pro 3,1 logic board may not even fit properly in a Mac Pro 1,1 case.
This is why doing a logic board replacement (same logic board for a dead one)...is certainly doable. But...in many cases even doing an exact logic board swap...is many times not worth it due to cost of the replacement logic board (can sometimes exceed or be close to the used value of the computer).
I did see 1st Gen MBs on ebay but who knows what you're getting . . .
This should not be a concern. If you purchase something...and it doesn't operate as described (let's say it arrives DOA)...then you simply return it. If the seller refuses...then you open a Paypal dispute. Which is why paying with Paypal is a "good thing" sometimes.
- Nick