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David,
I read a few Canon reviews for the 400D model and for astro folk were suggesting ISO 800 as the sweet spot. I did that for a year - improving focus, pointing and guided tracking particularly. Once I could do longer duration subs reliabily I asked folk with alot more skill than I and Humayun in particular suggested I go with ISO 400 shots.
One key thing I've just noticed in my darks is that at ISO 400 there is significantly less noise at 10, 20 and 30 minutes shots. In my equivalent darks at ISO 800 there was real noise past 15 minutes. Now noise can't be perfectly subtracted - so I think I'll stay with the lower gain of ISO 400 on my longer shots.
Hope this suggestion helps. It boils down to if your darks are truely dark at the duration shot you wish, and you're still getting plenty of signal - then you should be laughing with ISO 400. Try it and see.
Matt
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