According to Wikipedia:

Purchasing Power Parity (PPP) is measured by finding the values (in USD) of a basket of consumer goods that are present in each country (such as orange juice, pencils, etc.). If that basket costs $100 in the US and $200 in the United Kingdom, then the purchasing power parity exchange rate is 1:2.

So, PPP GDP is the “effective GDP” and this is the list of the top 20 countries by PPP GDP:

One can see from this chart that China is already ahead of the US, India is ahead of Japan, and Russia is very close to Germany.  Also, traditional economic powers like the UK and France are in 9th and 10th place, respectively.