[sanddrag]

While RC car motors are relatively inexpensive, easily available, and very powerful, they are extremely fast. You will need a lot of gearing which may not be cheap or easy. I recommend getting a gearhead motor like this that already spins pretty slow.

I'm not sure how big or small you want your robot to be, or how fast or slow, but you might also take a look at these motors http://robotcombat.com/marketplace_ant_motors.html