Shoebox SFFs like the Shuttles all have PROPRIETARY mobos. You can't replace/upgrade the mobos in them from anyone else but the OEM. Far better to do a standard microATX mobo in one of the new compact cases like the Enermax Venus series or the Athenatech A100 series.
. MiniITX mobos are the really tiny and generally underpowered ones from Via that techies have built into toasters, cigar humidors, model cars, etc. MicroATX mobos are about 10x10 inches. FlexATX mobos can be smaller but usually don't have any PCI slots on-board- you have to have a "riser card" with the PCI slots on it. Anything less than microATX is pretty much a waste of time if you want any significant performance. The shoebox SFFs are dead-ends so I don't even consider them.
.bh.