Ok, speculating in a different direction, the area for the CPU seems to have the standard mountings for some fairly heavy duty cooling, which a high-efficiency CPU probably wouldn't need anyway. So maybe we are barking up the wrong tree all together...
There are four memory slots. A machine with only a 32-bit address space probably doesn't need that many. A machine with a wider address space (either 64-bit or using some sort of physical address extensions) might do. What sort of PPC processors match that description?