Do we really need such accurate calculations for that? I would rather go for number of cores counted in dozens ;) We don't need accurate numbers but speed and having more bits won't do it. And even then you can have seperate processors with own memory just communicating each other as different parts of brain do.
And BTW characters can think that they're real even if they would be 2D or even if they would have some very simplistic senses and wouldn't have any virtual body at all ;) And this would be much more interesting than simulation of real world :)