Relatively regularly these days there are calls for a Manhattan or Apollo style project to address our energy challenges. While this may sound good, it's worth considering what such a project would actually mean. I found this article (pdf) to be helpful. For the Manhattan project, peak annual funding reached about 1% of total federal outlays and 0.4% of GDP. For Apollo, the peak annual numbers were 2.2% of the federal budget and also 0.4% of GDP. What would that mean in today's numbers? Well, the federal budget is on the order of $3T, and the GDP is around $14T. If we use the GDP numbers, such a commitment of resources would be $56B. That's approximately the combined budgets of DOE, NIH, and NSF. Bear in mind that when we did Apollo, it's not like all other efforts stopped, so really pulling something like this off would require a significant allotment of money.
It's worth pointing out that the government has given three times this amount to AIG alone. It's also worth mentioning that communications technologies are vastly superior to those of the past. Presumably large collaborations can be managed more easily and would eliminate the need to uproot the top researchers in the world from their homes and relocate them in a single central location.
Anyway, those are at least some real numbers, and they're not crazy or unattainable given a strong lead in national priorities from the top. The real challenge is figuring out what the true goal is. It's fine to say "energy independence" or some target number for renewables, but the global energy challenge is a lot more diffuse and multidimensional than either putting people on the moon or developing the atomic bomb.