if you want good ISO 800 you need a D-SLR (and not all of them - e.g. Olympus, Kodak aren't good at 800).

Higher ISO = less light = more noise = can use higher shutter speed for a given aperture (so if someone is moving, it can eliminate blur).

If you combine a good low light SLR (e.g. ISO 1600 or even 3200) with anti-shake (especially in-body like Sony and Pentax) and a fast lens (e.g. 50mm f/1.4 prime or Sony 28-75 f/2.8 zoom), it means you can take pictures in low light without flash.