The world is overpopulated? News to me. The way I see it, it's under-producing food, largely because of irrational resistance to modern techniques.
While I do agree that the planet can sustain a much higher population, modern farming practices are causing problems and not necessarily sustainable in the long term. (http://en.wikipedia.org/wiki/Intensive_farming#Disadvantages)
Properly thought out food production that utilizes under-used resources (vast swaths in Russia for instance!) is the key.