I don't know how I missed this. China and India have low GDP per head, although they are getting richer fast so therefore the eclipse of the USA seems inevitable ... but the USA's population is growing fast, and China's and India's are not. Considering how vastly richer the USA is right now, and considering that this population growth only grows the economy further, it seems clear the USA will remain by far the most powerful country for a long, long time.

“The population"s of Europe, Russia and Japan are declining, and those of China and India are levelling off. The United States alone among great powers will be increasing its share of world population over time,” predicts Michael Lind of the New America Foundation, a think-tank. By 2050, there could be 500m Americans; by 2100, a billion. That means America could remain the pre-eminent nation for longer than many people expect.

http://www.economist.com/world/unitedstates/displaystory.cfm?story_id=15108634