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Assuming equal skill and care during exposure, I think you'll find that the answer from most people is "anywhere from 1 to 2 stops" as you noted. I never owned nor shot a D3 or D700, so I can't really opine directly.
But I've made some nice, clean images at ISO 12800 with the D3s... that camera really made me lose my fear of cranking the ISO up when necessary. And note that you do need to learn high-ISO technique to get the most of any of these cameras, OK? The absolute Kiss of Death is underexposure. Better to raise the ISO another notch or two, but make absolutely sure that you ETTR, and you're probably in pretty good shape.