Isn't that a mini system? You should really consider all your options before limitting yourself in such a way. Shuttle mini systems have at most two slots (1 AGP and 1 PCI), chances are at some time you'll need at least two PCI slots. If you were thinking that USB was a reasonable substitute for PCI, thing again, USB has extremely low performance, things such as USB tv-tuners for example can't even do the job properly, and such devices are not available in USB 2.0. Besides, having external boxes laying around ones system makes the "conveniece" of having a small system...disappear.
The best type of case for a Home Theater is an ATX desktop, because styles are available that resemble most home audio components in both size and color. The best portable system would be a Micro ATX tower, which is actually narrower in many systems than a mini case. Mini's don't save deskspace because deskspace is a horizontal, not verticle measurement.
About the only practical situation for a mini-PC is as a portable gaming machine, but we already have consoles for that.
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>