Having a memory — RAM or disk — above 2^64, however, is not achievable in even in theory... 2^64 is only 100 times less, for example, than the estimated number of sand-grains on Earth.
I don't think its fair to say that it isn't even achievable in theory. If you could write a bit of data into every atom of a piece of silicon, 2^64 corresponds to 860 micrograms. I would start to agree that pushing much beyond 128bits is crazy as that puts us into the truly collosal range of cubic kilometers of memory Though there are things smaller than atoms...
That being said, even the most wildly optimistic projections about the rate of increase in memory would put such a piece of memory at least 20 or 30 years out.
"An ounce of prevention is worth a ton of code." -- an anonymous programmer