There's... really no one way to tell who will be the leading super power in 2050. Honestly, there is no way, absolutely no way to predict which nation will be dominant. Trying to make a prediction assumes that the current trends will continue for decades, and there's no real reason to think that has to be the case. For example, China's economy is doing good right now, and we should all applaud the fact that literally hundreds millions of chinese citizens have been lifted out of crushing poverty. While I hope for China's economy to continue to do well, there is no guarantee that it will. Remember the 80's? I sure don't, I was only born in the late 80's, so I have no recollection - however a lot of 80's films and books dealt with a "Japan" syndrome - a US fear that they would be dominated by "JAPANESE OVERLORDS!" and that Japan was going to take over all the American companies. Guess what? That didn't happen. Japan's economy burst in the very early 90's and it has remained pretty much completely stagnant ever since. I hope something similar does not happen in China (because I want more chinese citizens to be lifted out of poverty), but it could very easily occur in China. The Chinese government is actually quite nervous that their property bubble could burst, crashing the Chinese economy.
But I can wildly guess about the balance of power in 2050. I think China and India and Brazil are here to stay as powerful nations with influence. I don't think the US is going to collapse, nor will Russia resurrect the USSR. I predict a future in which China, India, the USA, the EU and Russia all play a big role in the shaping of world affairs. The UK and France will influence the world has a part of the EU, but let's face it, the glory days of the British Empire are well and truly over. Same thing for the French Republic. We're headed for a multi-polar world - you know, like most of the world has been for most of its history? It's only been VERY recent (last century or so) that one or two major powers have dominated global affairs. The dominance of the US was actually a serious historical abnormality.
Globalization will pretty much ensure that China, the US, India and Russia will all pretty much keep pace with each other. One side can't accrue a significant technological or organizational advantage over the other, as all the tricks have been learned and all the info spreads so goddamn fast due to the internet and jet travel. The world is becoming smaller and more "samey". Hell, I can't even tell the difference between the Americans and the Chinese anymore. Both eat McDonalds, wear Nikes, listen to iPods, wear business suits, drink coffee and tea, work from 9 to 5, drive to work in German or Japanese cars - apart from the language difference, I can't really see any differences between the life of an office worker in Shanghai and the life of an office worker in Boston anymore.