American Public Media, producers of such radio programs as Marketplace, Weekend America and Speaking of Faith (among many others) has a cool tool on their website right now.
As part of a special series called "Consumed," which explores whether the modern American lifestyle is sustainable in the long run, there is an interactive game designed to illustrate the impact of our lifestyles on the Earth. It asks you a series of questions about your lifestyle and, as you play, it will show you how many "Earths" of natural resources it would take to sustain all 6.6 billion humans if everyone lived like you.
Most of our friends think we’re weirdos (or incredibly annoying, I cannot tell which) since we belong to an organic food buying club, drive a hybrid, recycle and compost like mad, etc. Despite all that, my score was still 2.6, meaning it would take 2.6 Earths if everyone lived like us. (Curiously, the transportation section was devoted to questions about how often you car pool or use public transit. No questions about the kind of car you drive, other than its mileage.)