Like the D200, it is not bad at ISO 800, and sometimes fine at 1600 depending on light. In terms of noise, it's at least as good as the D1x, but the color is better. The D1H is almost one stop better than the D200 at ISO 800 and 1600. The D1H falls apart after 1600.
For an 8x12 print, my experience from least to most noise of the older bodies is:
D1H > D200 > D1x
That's with roughly 2/3 stop difference (noticeable increment) between each. The D1H is virtually noiseless at 8x12 even up to ISO 1600 at times, if light is good quality and the exposure is accurate.
And it seems the D2x is the same as the D200 at ISO 800. I have no idea how the D2H and D2Hs rank; I've heard and seen conflicting evidence, and it also depends more on IR level for those bodies because their filtering isn't very strong.
I get pretty good results using bounce flash with the D200 up to ISO 1600. It seems the D2x is within 1/2 stop of the D200 at that ISO level.
The D2x also appears sharper than the D1x and D200 at ISO 800. The color rendition is far better than the D1x at this level, and maybe better than the D200. I really don't shoot with the D1x in low light anymore for these reasons. The D200 does a bit better, and the D1H better still. The D1x is mainly my outdoor/good light/fast action camera of choice due to the vertical grip and fast AF. The D1H gets used for lower light action.
Of course, this about what you get at 12,800 with a D3s. About 4 stops higher than the D200/D2x with the same noise. To me, 12,800 is pretty much the level I can't see going above frequently.
With a D200, underexposing ambient light by 1-2 stops at ISO 800 or 1000, and using bounce flash, buys me better image quality while keeping the image looking like natural light. Just a little trick for using the older bodies in really low light without resorting to really high ISOs. I have, of course, maxed out ISO on occasion in really low light or when this technique isn't the best choice.