Here is a link to a review of the Canon S5:
Look at page 7 of the review and you can see the noise level at various ISO settings.
400 is not that bad.
800 is bad
The reviews of the S3 and the SX100 look almost identical to the S5.
The SX100 at 400 is better by a whisker.
I'm trying to decide between these three cameras but I am finding problems with each of them and would go with a dslr but don't have the money for the lenses I want so the slr would be pointless. The one at first glance that looks most intriguing is the Canon s5 is. Great feature and the size doesn't really bother me that much because I like the zoom. The thing that is holding be back from purchasing it is some reviewers have said because it is an 8 megapixel camera, there is a decent amount of noise even at iso's as low as 400. With the canon s3 is, I like how it has just the right amount of megapixels but I really dislike the small lcd screen and the lack of face detection and digic 3 processor. With the sx100 I like it in every way except for the lack of an electronic view finder and the delay some people say it has when using flash photography. 4 seconds between shots is not very good. so i'm stuck. thanks for any help you can give.