You need to worry more about performance when using a slower shutter/lower iso. For me it's the opposite although needing iso 6400 or higher is rare.
Why do I need to worry? I rented the D3X a long time ago and the low ISO performance was very good. ISO 100-400 was similar to the 1Ds III until you go up to 800.
I finally looked at the Flickr samples at ISO 3200 and 6400 on my good monitors and they look pretty good for full rez jpegs. The D3X at those ISOs were very noisy in comparison to the D600 which I believe is an improved sensor.






