Great stuff, thanks.
If we're talking about jpegs direct from the camera, can we throw the improvements between ISO 50 to 100 out the window?
From looking at the noise in the various ISOs from the RAW files with no noise reduction during ACR processing, I doubt you'll tell any difference between them. It wasn't a huge difference between 50 and 100 but it was clearly visible at 100% (pixel peeping...). In fact, in JPEGs directly from the camera, I doubt you'll tell much of a difference from ISO 50 to 400! 800 looked like it will clean up with minimal loss of detail too. Above that, the noise level starts to ratchet up signficantly with each full ISO step. I will say though that the noise remained in a "tight" pattern all the way up through 12800; it just gets heavier as you go up but maintains quite a bit of detail. At 25600 you start to see some of the random large chucks of noise that really impacts small details in the image and I wouldn't even bother with H1 or H2 unless you're desperate for any image at all.
Remember these comments are based on the Imaging Resources RAW files shot with studio lights. To me they represent near "ideal" conditions. Out in the real world, any underexposing of images always results in greater noise...