This is a poor request.
1) We don't know what you want out of a PC in the car. Do you want to play MP3s? GPS navigation? Control of your cellphone? OBDII interface? A mobile office on wheels? WiFi access? Figure that each function you want going at the same time is going to eat up a little more system horsepower, so the more functionality, the higher the system specs you need.
2) If you can't even come up with a ballpark for system specs, you're goping to have a very rough go with this project. It's not as easy as buying a Dell and putting it in your trunk.
3) You say you've searched, but there's a whole forum of nothing but FAQs, including at least one about system specs. It's called the FAQ Emporium. You either didn't search hard enough or this forum was somehow overlooked. Go there and read.