And as you will hear over and over again, unless you do 3d rendering while driving you wont need a P4 and you definately wont need a video card. It will cause you more problems in the end then it is worth. Pentium M or Core 2 duo's are good power efficient and beasty cpus.
If you already the P4 lying around as a spare, then go ahead and use it. Just be warned. A big heatsink in the car is no good. All that heat on cold days will mean condensation in the winter which could be very very bad. Also the power supply you will need is going to be bigger.
There are many other options for power supplies. Opus 320 and DS-ATX are probably your best bet. Netiher available in the store. The opus 250 I believe is.