ISO 100 is sharp, detailed and free of any image degrading noise.
ISO 200 is almost identical to ISO 100 save for a minute loss of shadow detail.
ISO 400 is noticeably softer than ISO 200, although you really need to be viewing the image at 100% (as above).
ISO 800 is the cut-off point, whereby noise begins to visibly impair overall image quality.
ISO 1600 is fairly soft with a fairly prominent loss of shadow detail.
ISO 3200 is the top setting and while it will help you to capture an image in dim light, quality isn't great.