I was wondering about this topic for many years. I always set my web graphics on 72 dpi. Some time ago there was an extensive article on photo.net proving the dpi feature is literally meaningless. Pages over pages, many photos, documents and screen shots. Don't have the link anymore. But I found this instructive article:
http://www.photo.net/learn/resize/

Note that the ppi/dpi-feature can be important for prints.