For these comparisons, I'm using the full image. Again I'm showing the close up subject and the distant subject. I'm using Photomatix Pro 5.1.1 with the Painterly 3 preset. In my opinion, that setting brings out a medium amount of noise.
Close up Subject:
ISO 800 f/Stop 22 1/320
In this shot - I really can't tell much of a difference between the two. Experience tells me all the 'noise' can be cleaned up easily in both shots.
For the distance subject:
ISO 200 f/Stop 29 1/40
ISO 800 f/Stop 25 1/200
ISO 1600 f/Stop 29 1/320
As in the short distance subject, the 'noise' can be dealt with. Another characteristic I'm looking for is the detail in the ice formations. At ISO 1600, it is OK.
So for HDR purposes, I wouldn't be afraid to push the ISO to 800 or 1600 if I needed the aperture settings or speed.