How long will China reign as the World's most powerful country?

While that's certainly true, North Korea would just sign their own death warrant if they were to order a nuclear strike in the South. Would any nation support them? I don't think China would risk its relationship in the international community by endorsing anything like that. I'm guessing that if nuclear bombs were dropped on South Korea, several different nations, probably including the United States, would get involved.

@Liam & Mike

My impression has always been that North Korea's nuclear weapons were more intended for leverage against the United States and Japan than for use against the South. This is because North Korea really doesn't need nuclear weapons to cripple the South because the 38th Parallel has possibly the largest concentration of heavy artillery on the planet--and almost all of it is within range of Seoul. When you consider the number of guns, and the possibility that the North could use shells that carry chemical or biological agents, then I think you can conclude that North Korea's threat to turn Seoul into a "sea of flames" is not idle hyperbole. Unfortunately, much of the North's artillery is protected by armored bunkers atop mountains which are quite difficult to successfully knock out. So, Liam is correct that the South Korean military outclasses its Northern counterpart. However, it should be remembered that the North certainly has the capability to take down much of South Korea simply with conventional weapons--to say nothing of nuclear weapons.

China's economic power will increase for the next few decades, but then go on a downward spiral after that. The problem is that the Chinese had a huge population growth in the 70's and now they reduced the population growth. So we are going to have a lot of people retiring and all of them being replaced with a smaller work force.