I can confirm that the 867/1Ghz family motherboards are NOT compatible with the 1.33Ghz. I've used and replaced both of them (My mod for the GMMC this year was based upon both an 867Mhz G4 12 and a 1.5Ghz G4 12.) and the 1.5Ghz contain the support skeleton in which the motherboard mounts to (Or half at least), my 867Mhz did not, the motherboard screwed right into the case. The DC-Jacks are compatible, the DC to DC boards are not. Also, heatsink wise they differ.