Are there motor controler boards or instruments that can be controlled by the Stargate
Controller boards connected through a serial port are a good place to start. Here are a couple of examples. Some of these boards have H-bridges while others are designed for servos.http://www.acroname.com/robotics/parts/c_BrainStem.html
CMU developed a controller they call the Cerebellum which is being used in the Personal Exploration Rover. I don't know about its availability.http://www-2.cs.cmu.edu/~myrover/cerebellum/
There are also controllers that are connected through USB. Examples:http://www.phidgets.com/
A good resource is the advertisements in the "Nuts & Volts" magazine. The trick is to find a good set of software drivers.