[Selecting DC motor for robot]

Hi,
i am making a robot which would be having a weight of not more than 10Kg.it is a four wheeled robot with chained differential drive and has two motors in all.i want it to have a good speed and a good torque.can any one tell me which motor will be best for it,and y that motor is best??

[how this H-brigde is working?]

Please help me out to understand how this H-brigde is working?
i tried but i couldn't make any sense.
i mean i know the general working of the circuit...but i don't know the reasons why one transistor goes to Saturation and other goes to cuttoff...

i found it through google.it is attached in the form of image..
thanks