MichaelMeissner:
If you were just beginning the project, and were only considering using the Arduino to control the motors, it might have been simpler to use a motor controller with a USB input, so you could control it directly from the PC, such as Pololu Simple Motor Controllers.
Well it looks like LunchusBoxious will have to add extra functionality to justify using the Arduino... Like displaying the output level of each motor on a LED bar graph and/or LCD. 