Suggestions needed for motors & controller

Hi I'm brand new to arduino, and fairly recent to robotics.

I would like to run a bot with a fairly powerful drive system to carry 4kgs with relative ease. I have looked at the motor shield but dont believe that will be strong enough to handle the loads the motors will produce.

Can anyone reccomend a motor controller / motors that would work neatly with 6" pneumatic wheels and interface well with the arduino?

PS my ultimate goal is to have a bot with an arm that can pick up 1kg (although probably not at full arm extension).

The first thing to determine is what motor(s) to use. Once you know that, the choice of a motor controller will become more or less obvious.

Choosing a motor is a bit harder. There are direct drive and geared motors. Which one to use depends on how you will connect the motor(s) to the wheel(s).

How many motors are you thinking of? How will you steer the robot. How fast do you want the robot to go? On what terrain? Smooth floors require less power than rough, rocky, steep dirt.

How will you power these motors? How much space do you have for them?

I was thinking of differential drive. 2 wheels for now until I have the funds for 4. They will be powered by on-board battery. Considering the wheel size is known, i guess i will need to calculate rpms to decide upon a motor. The torque is slightly more complicated. I would like it to be able to handle moderate outside terrain (pavements [sidewalk], lawns etc.) As i'll be using 2 to start with, I guess that I wont need too much, the other 2 wheels will increase the torque later on. The bot will be steered via xbox 360 controller linked to PC, which is on a wireless network, which the arduino will connect to via the wifly shield. space is not an issue as the bot has not been constructed yet. Although they will probably be powered by lead -acid battery for now as NiCads are expensive.

Is there any advantages/disadvantages of geared/direct drive motors?

I suppose a medium duty MC would be needed, perhaps 2 (unless they have the capacity for 4 wheels)

Geared motors provide a lot more torque, at a slower speed, compared to direct drive motors.