I found this chart in The Economist to be astounding when I see the changes in GDP shares of the G7 versus the Brics. It’s striking evidence of the shift in purchasing power from the G7 countries (essentially “the West” or the First World) to Russia (the old Second World) and the largest countries of the developing or Third World).
Smart People I Know 
Another factor to keep in mind is GDP per capita. Even though China’s GDP will be likely be bigger than the US’s in the not too distant future, the US is not losing much ground on a per capita basis.
Graph: http://www.carnegieendowment.org/images/article_images/GDPpcR4.gif
Excellent point, Tom! This measure is just one of many that can be applied.