I don't know much about Cappuccino PC's so this advice is worth what you paid for it...
Generally, firewire and USB power is supplied by the computer. If you're using a USB hub, depending on the USB devices you have plugged in, it's possible you'll need a "powered" hub. But if you're plugging directly into the PC, it ought not to be a problem.
As far as the Xbox, unless you plan on running it off of the computer's power supply, you'll have to solve the startup and shutdown problem somehow.
You can purchase something called a Startup/Shutdown controller from mp3car.com to solve the amplifier "thump" problem. You can search on terms "amp thump" or the like to find other threads about this.