May I suggest, like "CDR Bill", that you use a phidget servo controller in combination with a motor controller such as this one, for example:http://www.parallax.com/StoreSearchResu ... controller
Ive used this before and its very easy to use. You control it just like you would a servo motor, except for the fact that you control the speed rather than position. Its also has a very short response time and goes from full forward to full reverse without any hassle(if thats something that you would like to do).
Its only 25 amps so you should test your motors stall current before you order any motorcontroller at all.
Good luck on the project!