If you want a guaranteed Q6600 G0, buy it from clubit.
They guarantee that the one you are buying is the G0 stepping.
Here:
Q6600 G0 Retail or this one
Q6600 G0 OEM
The difference between this G0 stepping and the B3 stepping (previous one before G0) is that the B3 runs hotter than G0. Not by a whole lot and it will only affect performance if you will be OC-ing it. Otherwise with a good heat sink, it should perform well enough for normal usage.