Torque helps your robot to accelerate, and to fight outside forces (most commonly gravity when going uphill, and friction); it does not affect your robots top speed other than fighting these two factors. You cannot calculate the top speed your bot will have unless you know the amount of friction. I believe I recall admin saying that a good rule of thumb is to get motors that have enough torque to lift your robot vertically. Just take the weight of your robot, and multiply by the radius of your wheels to get the required torque. If you measure in kilograms and centimeters, this will give you the kg-cm torque of your robot. The same goes for oz-in.
A helpful tool I use for comparing motors that are measured in different units can be found
here.
