There exists the possibility that both the US, China and Russia could be super powers - super power isn't an exclusive title - for 40 or so years there were two super powers, the USA and USSR.
Frankly, I think we are headed towards a multipolar world in which the status of "Superpower" won't even mean much. There's no doubt that, barring a major disaster, China is here to stay. But so is the USA. China's economy is rapidly growing, and with careful management it will continue to grow reasonably well - but as an economy gets bigger, growth gets more and more difficult to sustain. China's economic growth doesn't have to stop, but it will slow down in the future.
Personally I think China has already become an economic superpower. In military terms, it's quite strong, but still far behind the US - the Chinese have virtually no blue-sea capable fleet, although they are starting to built Aircraft carriers now, they won't have a fleet that can challenge the US navy for at least 2 decades. By 2050, who knows what the Chinese military is capable of.
China also have problems they are facing: An aging population due to the one-child policy which will cause many problems for their economy later on. They have decided to revise the one child policy into the two-child policy, which will come into effect later next year, but the "damage" has been done - they are facing an upcoming shortage of young people, much like how the US is facing a crisis with its retiring Baby Boomers. China also has a critical problem with its inflation - the Yuan should be worth almost as much as the US dollar, but it isn't due to the Chinese government artificially keeping their currency low. This has a benefit in lowering production costs, but the downside is that inflation is rampant and wages aren't keeping pace with inflation - people in China are starting to get VERY ANGRY with the government over these price increases. The Chinese government can't keep their money low forever, their own people demand an end to the inflation.
There is no doubt that what the Chinese People (and to some extent, the Chinese government) have achieved over the past 20 years has been remarkable. Almost an economic miracle. They've worked damn hard to get to this point and they've made some very smart decisions. And there's nothing that prevents further economic success, and I wish China continued economic success. I think it's a good thing that hundreds of millions of people have better, middle-class-like lives, and I hope China can continue to lift more of its own people out of poverty.
But China is not some invincible Economic Juggernaut that is destined to rule the world. No country is, not even the US (as the 07-08 crisis made clear). People also made the same mistake with Japan - remember all that talk of "Japanese Overlords" in the 80's? I'm not saying China's economy will stagnate like Japan's (at least I sure hope it won't), but that is a very real possibility. They have just as many problems as the US - in particular their pollution and desertification problems are far worse.
So Here's what I think will happen by 2050:
1) China and the US will both be economic super powers. The US is in a rough patch now, but it's always been in rough patches - there's nothing to say that the US is on an inevitable decline. Maybe an inevitable decline in terms of relative power, but not actual power. The US still has the most advanced technology and universities and those tech industries won't vanish from the US overnight (although China will probably build companies to rival anything in the US.
2) Given China's reluctance to involve itself in any sort of overseas dispute, the US will be the one to have global roles in military affairs. China could start sending its army around the world to get its way, like the US does, but historically and culturally China has never done that, nor will its people accept that sort of role. Also, China's army is geared towards defence, not aggression (except against Taiwan - they're always prepared for aggression against Taiwan). I suspect China's strength will continue to grow, but for it to be mostly focused inward, not outward.
3) Brazil and Russia and India will become powers in and of themselves, but I find it unlikely that they will manage to match the economy of either the US or China. Russia's recent economic resurgence is mostly dependent on Oil - once they start running out (and they will) I'm not convinced that they have other economic sectors that can pick up the slack. India's economy is facing a lot of problems - there's a lot of instability, pollution and a crippling water shortage. They can't go the China path of economic development, because India is a democracy. They instead have to pursue the "high-tech" approach, which will work but takes a lot longer. Unlike China, the government can't mandate progress - it has to occur naturally, and although India is progressing, the rate of progress is extremely uneven, more so than in China. Some of my lab colleagues are from India - and they tell me there are places in India which are amazingly modern - but right next door, there are places which still look like they're stuck in the 18th century. They seem to really love the Private Sector, but they have nothing but scorn for the government's capability to solve any problem. I suspect that in time India too will become a super power - but it will probably take a little longer in their case. India are also struggling against Maoist rebels and religious strife.
Overall - I envision a multi-polar world. I don't think the US will fade - it will just become one power among many. And I think that's a good thing. China and India are on the rise - I think that's inevitable, and something we should welcome. How can we not welcome the fact that millions of people are living better lives and eating better food and contributing more to the human species? China and India will start churning out scientists, workers and artists and all of them will help solve some of the pressing problems.
Don't be paranoid - China and India have no real desire to supplant the US. China doesn't want to play Global-Supercop - it wants to leave that job to the Americans. Also, China's biggest current and future market is the USA - they don't want to see the US economy sink. India also does trade with the US and sends their kids to US universities.
I think by 2050, the world would have changed and that both the US and China and India will all get along. Wishful thinking on my part? Maybe. But that's what I hope happens.