Courtesy of The Princess, I just took the Ecological Footprint Quiz. If everyone consumed what I do, we'd need 3.5 planets. My total ecological footprint is 4 acres for food, 2.5 for mobility, 4.4 for shelter, and 4.7 for good/services, for a total of 16 acres. The average American's footprint is 24 acres, but there are only 4.5 biologically productive acres per person available on the planet.
Gives you a lot to think about.