Hmmm...I was intrigued at one point by the population density issue and, manipulating my fingers and toes like an abacus, calculated that you could take every living man, woman and child, give 'em all 4000+ sq ft of personal space and house everyone inside a 1 million square mile area. That's roughly 6 billion people in the western half of the US, assuming flat, livable ground.
Disclaimer: I was one of those kids who had to stay in during recess to work on my multiplication tables, so let the caveat totally reside with the emptor!