There are a couple of ways of doing this.
The first is to use a seperate screen in the sunblinds or rear headrest. The second way is to have a pull out screen in a 1 DIN slot on the dash.
The latter comes in two different flavours. The first flavour is screen only and cost £79+ on Ebay.
The second flavour is the all in one singing and dancing unit. There is a HK unit at around £599 (last time I looked) that had everything in one. However, the major brands will cost upwards of £1200 (eg Panasonic Avic N1).
If you get a screen only device you need something to power it, and so you can pick up an extra 1 DIN unit that contains DVD, radio, tv, mp3 player etc. You could power it from a laptop in the boot if you wanted - however, you need a touch sensitive screen and I've only found 1 of those from the States - specially designed for it. They are about £300 and plug into USB and Serial and work exactly the same way as a mouse. There are some apps that have been developed for Windoze and Linux that will use these touch sensitive screens and provide a full sound system delivered from a PC (search for pycar).
There is another windows front end (cant remember its name) that will interface (nicely-ish) to any app on your PC within a frame which means you can put a GPS application on as well.
