It really doesn't matter how good a computer is. You can still use it in the car.
It mostly depends on what you are planning to use it for. Some people like high end computers so they can play games or something. Some just like the basic things (music, gps, etc.). If you want to do things bluetooth I would recommend getting something with a built in one or throw up the money for a dongle. My first car set-up was a Dell Laptop that I got for free from work because they were throwing it out. It had 1 gb Ram 40 gb hd. 1.8 Ghz Processor. It did me justice until I needed more hard drive space.
Make the computer fit your needs. Several people think 1 GB RAM is too much, but I'm fine with it.
But first off I have to ask. Why buy a laptop? That's a waste of a screen and they get hot sometimes.