a stepping indicates basically a different version of a processor. G0 is the newer q6600 stepping (the old one is B3). the G0 is greatly improved over the B3 stepping especially in heat output (hence improved overclocking). you can spot a G0 by looking for the code "SLACR" on the processor's IHS (the metal cap on the processor) or on the box.
sorry, veedub is right. it is correctly called a revision, not a stepping. i fell in on the common title. stepping actually refers to something different and i've tried to clarify before but no one listens