I am sorry it is x86 not PPC but I cannot finance OS4 compatible PPC motherboard development. But if you want an x86 system like this please show support. A few members talked about donations to help (100% of your donations would be subtracted from purchase price of course).
You don't need to make yet another motherboard, just consider the available PPC OS4 motherboards as you go. Make cases able to fit standard motherboard types such as the standards done in Sam, XE, Micro, Pegasos, and/or X1000, and we're good.
