I have an Acer Netbook and a USB GPS unit. Got the netbook for 300 from Staples and the GPS from Amazon for about 30 bucks. Got a car charger and laptop case from Amazon for 20 dollars.
Thats probably the best in car GPS setup you can have. The screen (even a netbook screen) is much bigger than any in-dash system you can buy, you can simply type in stuff to navigate instead of pushing a hundred touch-screen buttons, and if you find a wi-fi hotspot or have cellular service on the laptop you can even Google for stuff to find and addresses.
Been using that for a few weeks now and its great. Much better than the crappy Pioneer car navigator I bought last year.
Right now I have a Motorola Droid and thats even better.
If you do the Laptop I recommend Microsoft Streets and Trips, though I hear Google is soon going to have voice navigation on their web-based map service.