In my search for a low power consuming hardware, I came across a new platform developed by Nvidia called ION. It's a mini-itx form factor motherboard with GF9400M video on board and takes either a single or dual core Intel Atom CPU. The first company to sell just the motherboard is Zotac. There are a few different packages to chose from, but this one looks like the best choice for a car PC.
Onboard Video GeForce 9400M
Onboard Audio 5.1
CPU Socket 441-ball
FSB 533 MHz
Dimensions 6.7" x 6.7"
Memory Type DDR2 667/800
Package Contents 3 SATA cable 1 SATA power cable 1 WiFi antenna 1 WiFi bracket
Memory Slots 2×240pin
SATA 3 + 1 eSATA
PCI Express x1 1 (mini)
Form Factor Mini ITX
Video Ports D-Sub + DVI + HDMI
USB 10 (6 on back panel, 4 via pin header)
Power Connector 24 Pin
Looks like this board can be had for about $220 here. Considering the CPU and GPU and Wifi are included and the size and effeciency, it seems pretty reasonable.
Nvidia is claiming to do full 1080p video decoding with this board and I believe it. I have a home media center that I built using a GF9400 and I can do full 1080p with as little as 10% CPU overhead. That is using an AMD 5050e CPU which is faster than the dual core Atom, but not by so much that I would expect the CPU usage in an Atom based system to be that much higher. I'm hoping the CPU usage will still be under 30% when decoding 1080p.
As for mp3's, the dual core Atom is fine. My only concern is how some of the bigger GPS software packages will run on a dual core Atom. I suspect this too will be fine given that most portable GPS units are running on 400mhz ARM CPU's.
You can also get a version of this motherboard with a built in 90W power supply and AC-DC power brick. This would be great for building a tiny media center PC for the home, but for a car, I'm looking more at the DC power supplies with ignition control to manage the system startup/shutdown a little more elegently than just cutting power when you turn off the car.
UPDATE: PCCyber has this board in my area now for less than anywhere else in Canada that I could find!