These Shuttle X cases are usually made for Mini-ITX and they have one AGP and PCI slot. Some has a PCI-E and PCI slot. Some have only one PCI-E slot. They suit one CD drive, one hard drive and maybe one floppy drive. Two sticks of RAM are usually there and two or three fans are in it. The PSU is lower on Watage, but so is the rest. AMD and Intel are two great PCs and they are better in terms of spacing, but some can have big guns in it. But, it will depends on the PSU wattage.
Next, Nano ITX and Mini-ITX are also in the VIA C3/C7 range. They usually rely quite on small stuff. One chipset, some has SO-DIMM instead of DDR. The power connectors are different, as smaller pin connectors, but there's cubes for these.
In fact, Cube PCs are great, but if I buy one and put like a ATX board...have fun! Some cubes MAY handle a Micro ATX, but I may be wrong. I usually see them in Mini-ITX and Nano-ITX. The Nano-ITX can be costy...$200-400 with a WELDED CPU. Mini-ITX may be better with a CPU socket and all that. Usually, the boards have all intergrated. VGA, Audio, etc.
read a bit on this and take a look on the board: