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Glen, you might want to try ISO 1600...the read noise is almost halved on the 450D, and since you're cooling your sensor anyhow the thermal noise should be well under control. That claws back much of the intensity you lose from f/5 to f/8.
I was recently lured into trying 800 with my 1100D, but the noise at 1600 is only ~30% higher for the same exposure time, so much of the doubling of signal comes for free. My main issues at the moment are thermal, as the weather is warming up. It's easy enough to use employ an intermission between subs of 60 seconds or so to give the camera some breathing time, and still accumulate data quicker than at ISO 800.
Of course, there's the question of dynamic range...which I don't fully understand, but I'm not convinced either way that it makes a difference from my usual imaging haunts.
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