I have since sold the DMR (I moved to medium format digital), but ISO 400 with the DMR is fine. It does not have the noise-free look of the ISO 100, but it is still quite good. It looks better to me than 640 on the M8, but worse then 320. A lot of it depends on exposure...high ISO with the DMR really depends on actually having light. So 400 looks good if you have a properly exposed shot, but if you just put it at 400 and then underexpose one or more stops, it starts to get noisy. Bluntly put, the point of the DMR is to shoot at 100. Anything less, and you are beginning to compromise its utility. Anything over 200 and it starts to lose any advantage it might have over the current crop of DSLR's. I also use the D3, and while the DMR gave it a run for its money at base ISO (showing about equal resolution and good color), the results gave way pretty quickly. If you intend to shoot a lot at even moderate ISO's, I think you would be better served by a D700 -- it is cheaper, smaller, lighter, full-frame, has a much better screen, and the high ISO performance is dramatically better. Most of the lenses are not as good, but some are really excellent (the new 24-70 was as good as the 28-90 ASPH), and you can always get the Zeiss ZF lenses if you feel the need for manual focus high quality optics. They are not as good as the R lenses, but it is angels dancing on the head of a pin in this case.
Anyway, this is just my take on it. Others will surely have a different opinion.