The P4 is typically one hot beast, and can be kept stable in cooler climates. On the the other hand, a 1.2Ghz dual core will be suitable for what most people need.
Dual core 2.2Ghz is on the high end for a car pc but it's something I chose for my project. So, it all depends on what you plan on doing in your car. Is the P4 a laptop configuration?