Though I expect this will be a long process I think we have seen the fall of Rome once again and the world will now begin to re-balance. Which countries will be on top I cannot say, the obvious choice in my mind would be China but a large scale world war could easily change that.
It would take quite allot to take down the western world and if it did happen I wouldn't call it a downfall. Maybe the lines between the western and eastern worlds wither away through then the western world will slip away into the sands of time. The reason that Rome fell was because of there vicious territorial ism and there far too big empire. One government cannot control 20 country's. America will it will most likely not fall dramatically.
This specific collapse? There has been collapeses of the united states economy periodically since it became an indepedant nation and the country has never blown up as a result. Its okay to just be kind of wary but maybe it is good to be just a little bit less paranoid.