20D may be an old camera now but it takes great shots and the high ISO rendering is excellent. You can certainly use ISO 800 with confidence and 1600 is very reasonable and will clean up well if necessary. Sometimes I use a Gitzo walking/monopod stick if light levels are bad and this can be used together with IS. With a long lens I use 400 ISO as my default setting even in good light and put it up to 800 or 1600 as necessary. Far better than the old high speed films!
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