Need assistance with potential Arduino bot

First off I apologize if I broke a rule by putting assistance in the title. Now about my problem: I am putting together a parts list for a Arduino version of the Start Here robot. I am not sure if I need a motor controller shield for the Arduino board. I am going to use 2 4.5v DC motors and a ultrasonic rangefinder sensor(5V according to yourduino.com) and I don't know if the Arduino Uno board can handle the power requirements. I was going to use a 6AA power pack to power the whole thing. If you notice any problems about my plan please tell me and if you could answer my question that would be great!

You are going to need a motor sheild
Unless this is a special arduino that has a built in motor controller.