Right now, I'm building a robot that is supposed to go up and down stairs. It will have wheels on bottom of the sliding parts. The modified servo that came with the VEX Robotic Kit is very slow. I don't have enough money for faster motors. The small geared motors like the ones sold by Pololu cost $16.00 or more but I'm not sure if they are powerful enough. I have two motor controllers sold by Pololu.
http://www.pololu.com/catalog/product/994
http://www.pololu.com/catalog/category/115
http://www.pololu.com/catalog/category/116
http://www.pololu.com/catalog/product/1372