When I first got my MDD I got 2 of them. One was a FW800 dual 1.25, and other was standard dual 1.25. Of course I hooked up the FW800 first, and it worked, but constantly gave me problems, freezes, and random resets. Sometimes it would refuse to boot at all, and when it did it only would show part of the installed ram.
After a lot of trial and error, I tracked it down to bad ram slots on the motherboard. I'm guessing somebody in the past put cheap ram in there, that was a little bit too thick, and stretched out the ram slot pins. Anyway, I had to junk the Fw800, nothing I tried would bring the slots back to use. I ended up transplanting everything into the standard MDD and its been working perfectly ever since. I recently upgraded its video card to an apple 9700pro I found on ebay.
I miss the FW800, with its bluetooth, and airport extreme.