That is a very subjective question. I have a D300 now but used the D2X till November, 2007. I adjusted the aperture to keep shutter speed high enough so I would not have to go above 640. I did not like going above 400.
With the D300 I don't mind going to 1200.
What are you shooting now? What iso do you need to get to?
I would shoot as ISO 400 or lower whenever possible, but would willingly use ISO 640. ISO 800 was a last resort, and if the image was underexposed at all, it was pretty much unusable.
I never used the D2x since I just converted from Canon a year and a half ago but do remember my friends using it being nervous at anything over 320 !!s
btw keep in mind the words of Chas ...... noise beats unsharpness any time !!!! :) Worry about getting a sharp image then look at your ISO