The fact is, they already exist for PPC and x86 processors (among others). This sort of thing has been done using SBCs for quite some time. They're usually either packaged as expansion card (think bridgeboard or
InsideOut) or in a 5.25" drive bay (the Index
Access Amiga 'clone').
I don't have any current links, but you can probably find them if you search for "SBC", "single board computer", "PMC", "PCI mezzanine card", and "PPC". These computers are primarily used in embedded systems, but they have been repackaged as consumer products in the past... Though typically for x86 processors, not PPC.
