The hardware for this project had to meet some requirements:
I chose the Via Epia with integrated everything. It has a low-power
consumption Via C3 533MHz chip that doesn't require a CPU fan, yet still
handles the decoding easily. The chipset has integrated video, ethernet,
analog or digital audio, and s-video, and no proprietary kernel tainting
I found a company called caseoutlet.com that sells the epia in a case that takes 12V, so I could wire it straight to the car if I desired. And by having the power supply external (if you go the DC->AC inverter route), it produces less heat in the box. The case is also quite small, fitting under my seat easily.
And most important, the Epia is priced similarly to normal PC parts, unlike most tiny machines. For <$200 you get the board, box, and power cable, compared to the >$500 I saw for 133MHz 586 SBCs.
For the hard drive, I went with a 30GB 5400RPM drive. A 5400RPM drive has more than enough throughput to play music, but produces less heat and noise (both electro-magnetic and audio) than a 7200RPM drive.
I bought a 128MB stick of RAM (120MB of which are routinely free). Be careful if you use this case! There's a CD tray directly above the memory sockets, which won't fit back in if there's a stick of normal height RAM in the front socket. So use short RAM or a single chip.
I chopped and twisted my ribbon cables, because the fan is directly behind the hard drive bay, and the cable was reducing air flow.