With a DC motor you control the current in order to control the speed of the motor. The usual way to control the current is to vary the voltage - perhaps using the Arduino analogWrite() function to control a Pulse Width Modulated power supply to the motor.
Great thread. I'm a newbie. I would suggest 2 things:1. Get rid of the power talk posts and include it as links (as you did with measuring torque). As mentioned, we are interested in making the stepper move.2. Can you make this and many other topics in this forum into stickies? I feel there are lots of common n00b issues that could be dealt with if stickies were available.