It shouldn't be that much of a stretch to recompile OS4.x to work on a x86 motherboard. The graphics cards are already used with PCs/Macs/Linux boxes, USB is USB, etc.
The speed difference between the fastest PPC and a middle of the road x86 is staggering. Apple dumped PPC for many reasons, but the biggest reason to switch to x86 was the performance gain. There won't be any faster PPC's made, and such is not the case for the x86 family.