Hi Rob,
I think the optimum ISO setting for DSLR's is between ISO 400 and ISO 800. That being said, I always use ISO 800 with my 450D and only use a lower setting on brighter objects such as the Orion nebula.
Like Humayun has said, always take dark frames. I like to use ICNR darks as this is the best possible way to match the darks to the lights.
