Most of those systems you'll find (and I've done my research, trust me) have older Core 2 Duo or Pentium processors, however, if you want to go down that route, go for it. Generally, if found in bulk, it is old school computers being tossed/sold out. Craiglist will be your friend in this case :D

Anybody who doubts that mainstream PC sizes are a function of underlying form factors should harken back to the AT and Baby AT designs of the late-'80s through mid-'90s. These 386-era behemoths were immensely popular but saddled designers with certain problems, such as a large chunk of the motherboard resting under the drive cage. The smaller you tried to build the system, the less convenient the machine became to work with. Killing boards by not inserting the power cables correctly was another minor problem.

When one talks about media center PCs, the assumption is that one means a mini-tower in an office setting and a CE-friendly desktop design in the living room. Strictly speaking, these aren't really small form factors, although the sleek styling from case manufacturers like Antec and SilverStone sure make you think twice. Fortunately, microATX does allow for some size shavings beyond these dimensions.