A window motor driving a toughbox with the second stage of gearing removed would be just about right to run an arm.
We thought about it a while and decided to use a V belt and pulley system, driven by a window motor, with a latex tube supporting the weight of the arm. Seems to work so far....but this is the prototype, not the finished product. One interesting thing about using a window motor with a geared or chained connection to the arm, is that you have to remove the robot from the field after the match, and if the arm is pointing up it's hard to get the arm back down. The belt lets it slip. Adjusting the belt tension so it slips when you want, and not when you don't, can be tricky.
http://www.youtube.com/watch?v=3vY2_h2GBIs
