I had a play with a GPS-augmented version of Autoroute some months ago. It was quite accurate at showing me where I was, but of course doesn't have all the clever things like dynamic re-routing and TMC which the better in-car GPS systems have. But unlike such systems, you can have a route with many waypoints rather than just a database solution.
However:
1. Laptop display too dim by day.
2. Laptop will need a power cable and GPS connection (unless you use Bluetooth).
3. Laptop speaker may be too weak - you'll need an external speaker. It too might need a power cable.
If I could do without the car for a few days, I'd have the current Bose set up removed and a Blaupunkt TravelPilot Rome installed.