I used the Tamiya Tractor kit which came with three motors and two gearboxes.
Because you are using analog motors I had a harder time researching code as most people modify the tamiya to use continuous rotation servos.
I also purchased a parallax ping sensor. I was able to get everything running in an autonomous movement setup. The ping sensor, or my coding is a bit iffy. I probably need to mount my sensor up higher in relation to the chassis.
I'm running mine with an arduino motorshield, and a Arduino Mega ADK, with the eventual goal of a controllable robot by 3/4G android with a wifi netcam mounted.
Check it out at www.murphyslawwrobotics.blogspot.com