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You need some 'ISO' to amplify enough photons to get an image to work on. If you imaged at 100 ISO eg you would probably need about 20 hours of stacked images to get enough photons to work with. By going to 800 ISO eg you amplify the signal the photon creates at the sensor and it takes 1 or 2 hours to get enough. The crunch comes in balancing the two. Too much gain = too much noise. As I mentioned before there are some people who have experimented seriously with various cameras and sensors to figure out the best option for each. Newer cameras have better sensors with lower noise, you can therefore push them to higher ISOs without too much noise being included.
My SONY SLT A77v has an excellent sensor, I have pushed ISO to 6400 but it's sweet spot seems to be 1600, maybe 2400. For older cameras like 400D and my 450D 800-1600 is about it. 800 is generally better.
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