I have done someting similar to this, based in the UK, I have set up my laptop with a wireless card, GPS, and WebCam. Then I created a Ad Hoc network to my friends laptop in his car with a similar setup.
We were able to have full duplex voice conversations for the whole journey, using Microsoft Netmeeting (difficult to find in XP). With My web cam we were able to do video conferencing as well, but that was distracting.
Gear used 2 Laptops running XP, 2x Orinoco Gold cards running @ 128Bit WEP set up as an ad Hoc netowork, Logitech QuickCam Pro3000 WebCam.
Range was good upto about 10 car lengths using built in antennae. I tried my home built cantenna (~10db gain) and that greatly imroved the range possible.
We didnt have Internet access as my phone dosent do data (Will get a Bluetooth GPRS phone soon)