Funny you should ask . . .
The night you posted this, I started work on my next tutorial - a step-by-step how to build a robot, optimized almost solely for simplicity and low cost.
I didnt reply tho cause I was hoping others would offer ideas up, first . . .
For the long answer, you will have to wait and see . . .
But for the short answer:
An AVR Atmel Microcontroller, 2 HS-311 Servos, a NiMH 6V battery, HDPE, and two photoresistors would be part of an ideal robot for what you ask . . .
look for each on this page: http://www.societyofrobots.com/robot_tutorial_index.shtml