At ISO 100, you'll be hard-pressed to see digital image noise. This is the setting of choice for landscape photographers who want to make large prints (the more you enlarge a print, the easier it is to see noise). At ISO 800, it will be easier to see some grain in your photos. The higher your ISO the more noise or grain you will see.

When enlarging you’re your pictures you have to bear in mind not all digital SLRs handle noise in the same way, and some are better at reducing it than others. Depending on your camera you may not see any noise at ISO 800, when enlarging a photo.