I purchased one of the SATA drive units (160gig I believe) with the extended battery for a similar purpose. I did it so it could be moved between cars for road-trips and such. I'm in the process of machining some swing arms so that the laptop can be mounted (kinda like a cop car's is). I use an aux input to my HU (pop-up DVD Player) and have it currently setup to use the HU's display as a secondary monitor. I have GPS software on the laptop which works really well (better than our garmin stand-alone unit). I figured out how to connect it to my cell and use it as a modem (480 up/down) and having internet on the run is sweet.
I always have the laptop with me but I don't always connect it - which is why I recommend keeping your head unit for radio and jacking in when you want more. It's nice to have both and I typically use it on the longer drives.
I do plan on using a touch screen as a second monitor in the next couple of months but finding the mounting location will be tough (I have a Dodge 2500 quad cab) since I want it to be located above the steering wheel to the left to minimize the distance I have to look.
So far I love it but keeping it simple really turned out to make it even nicer. I wanted to have a self-contained PC and I may some day. For the short term it really is awesome though!