Guess it also depends on how closely you want to simulate reality.
A friend of mine just finished his doctor-degree in physics.
For 6 yrs he tried to compute how the electrons move throughout a ~150 atom molecule (I think it was a small protaein). He told me that even though he had access time to (and used alot of) one of the largest super-computer clusters in northern europe, he could only make very crude predictions of the movement of the electrons, and then only at a small part of the molecule at a time. On top of that his work only simulated one aspect of this molecule.
Problems like this grows exponetially with every new atom you add to the problem...
Though alternate realities can always be simulated...
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