I am in the same boat with batteries. my solution so far has been to use three battery boxes for three M motors. I use axles to connect the battery boxes, and another axle though the holes on the switches of the boxes so all three can be turned on at once. It has much more torque with three M motors but still stalls on some hills. My next purchase will be two XL motors and IR reciever/remote. Currently steering is "leashed" with an old 9V battery box and motor.
As an aside, I created a brickshelf account this afternoon, but when I try to view the images it says the haven't been made public yet? Curious.