I routinely image in the "drunk or desperate or both" and "noobies on a rampage" range and I don't do so badly - cheers!!
That said, my best results always coincide with lower ambient temperatures, but that would occur with all iso settings and the temperature variations of the seasons.
I can only offer an anecdotal approach (from experience) to this in that I find iso1600 the sweet spot for astro imaging with the 40D - lower temperatures and many subs being the winning factors.
My mantra is "experiment" and try to push the envelope whilst doing it! I could never go back to 400 or 800 especially with my f4 lenses.
Depends on how much time one wants to devote to an image - if you want it all done in one night, then low iso will get you a smooth image with a certain amount of data in it. I favour the 10-15min subs at high iso over multiple nights (20+ subs) approach which really opens up the capabilities of the DSLR.
Great exercise for showing the 40D noise profile, but "happy medium" isn't where I want my images to reside
All the best.
Doug