Here is another very informative page with lots of examples covering the full range from 1 megaixels up (and film equivalents):
http://www.clarkvision.com/imagedetail/scandetail.html
Illustrates how much detail you can see at different resolutions and how the different film formats compare. The "master" picture is a 4x5 (large format) FILM image, equivalent to 200 megapix!