I was going to build a PC for my car but the one issue I have is the PSU. I was thinking about how I would power the SATA harddrive in the system. It would only be the motherboard and the harddrive that requires power. I was looking at three DC-DC PSU's on this site the M2-ATX, OPUS 120w or 320w. What would someone suggest I used to power my system? Here is what I will have.

JetWay JNC92-230-LF Intel Atom N230 1.6GHz Intel 945GC Mini ITX
OCZ 30GB SATA II SSD SATA II Drive
1-2 USB non-powered devices(maybe)

Thanks,
Kyle