If you go with a PC solution, get an Opus power supply.  I'm running Windows XP on an Epia M-10000 motherboard (1ghz, 512megs RAM, 250GB storage) in my car computer.  I use a 90watt Opus power supply, and it is fantastic.  Built in regulator, automatic startup/shutdown.  It puts my computer into hibernate everytime I shut off the car, then automatically brings it back up.  It can also use Standby if you want.
I know you keep saying you need lots of CPU power, but you're trading a lot for it.  Size, battery life, money, etc.  I do constant realtime video recording, audio playing, movie playing, media management, GPS, and wireless network scanning from my setup, and that's with a little 1ghz processor.