Hi,
If you do not want to build the electrical board you can purchase the Roboduino - it has all the capabilities of the $50 Robot Board and more ! It uses the same main microcontroller as the $50 Robot Board that you would have to make.
I'm working on a modified tutorial for using the Roboduino with the 50 dollar robot tutorial - either wait two weeks for the tutorial or email me and I'll walk you through the stages ( my email is airman00 at gmail.com )