No world power can stay on top forever. Eventually, even the strongest of nations, like the United States of America, will succumb to the inevitable. On average, with few exceptions, nations that emerge as world powers last about 200 years. We have lasted a bit longer than that and if we don't get our act together we won't make it to 250.
China, on the other hand, is one of the oldest empires in existence. They have had times of great prosperity and world dominance and they have had times of decline and deterioration. They are currently on the rise politically, militarily and economically. Some experts believe that China will surpass the United States as an economic power in the year 2016.
It is not a matter of "if" China will surpass us, it is really only a matter of when. They will have their run as a dominant world power and eventually some country or empire will overtake them. It is a cycle every empire goes through. Some, like Rome, lasted 1200 years. Some, like Germany, lasted a couple of decades. Future historians will look back on the United States' history and refer to us as a "flash in the pan" empire. Unless, of course, we can overcome the obstacles we are currently facing, return Freedom to the People and return to the process of private industry and government answering to the People rather than the other way around.
China is strong and growing stronger every years. America, on the other hand, is growing weaker every year. America is going bankrupt and businesses are going out of business. The government can't get it together and is always at risk of shutting down. China is thriving and strong and will probably overtake the U.S.
While China economy is growing at a substantial rate, the United States economy is still "much higher on the food chain". The United States has been an economic superpower, along with being a military superpower for some time now. As military spending goes, the United States' spending constitutes 39.0% of all of the worlds military spending. This easily makes us the most military heavy country on earth. Along with this, the United States itself is more resource heavy than China's. A lot of China's economic boost also came from the building of "ghost towns". Massive urban jungles built to fuel the economy and give workers jobs, and then never bought once the project was finished. All in all, I do not foresee China overtaking the United States in the near future, if at all. Source for the percentage: http://www.globalissues.org/article/75/world-military-spending