what you're describing is what Windows 7 offers in the way of Hybrid sleep. It puts the computer in to sleep mode, but saves a hibernate file first, so that if anything happens during sleep to make the computer turn off it comes back from hibernation. The problem that you'll encounter is having a safe trigger to turn your carPC from sleep into hibernation. Now normally with a standard PC unless you get REALLY tricky, your car battery is your trigger. It will shut down the PC when the voltage requirements of the PC become too much for the battery to withstand. Course when you get to that point you have bigger issues because your car wont start.
With a laptop, you do have a secondary battery that comes into play, and that battery is a lot safer to use as a trigger, but you will still have to do something to make sure the laptop charges when the car is on and goes into "battery" mode when the car is off. This could be something as simple as using a switched cig lighter.