Shatari said:
China is already very close to being the largest superpower, since they have plenty of nukes to play with, a massive (and surprisingly nationalistic) population, a decent stockpile of conventional weapons, and a half decent infrastructure for mobilization. They're big enough to scare the spineless UN forces (just like the US is), so I'd say they're pretty up there on the power scale.
They have a good few nukes to chuck around if they so wish, but by estimates they have less than France. In that respect they're still small fry. As for the massive population of China, this really seems to be more of a hindrance (in the long run) than anything. That they are sometimes embarrassingly nationalistic really only highlights the insecurity felt by people. Much is made of the former glory, seemingly in an attempt to hide the inadequacies of the present. And the armed forces of the PRC, whilst improving, is still a creaking hulk of a thing. It struggles with endemic corruption just as much as any other department of the central government.
As others have pointed out, there are many conditions that need to be met for China to rise to anything even approaching the U.S as is. I don't think 40 years is enough. China (read: the administration) will probably grow richer etc. but I think what marks the USA out from other potential superpowers is the quality of life it offers its citizens. In that respect I doubt China will ever be able to emulate America's success.
Edit: Of course these are all merely the uniformed ramblings of a petit bourgeois. All hail the great helmsman and party as they steer the nation to prosperity! etc. etc.