theres an article below that points out quite clearly that a computer capable of addressing memory through a 256 bit capable memory address space, would have the capacity to contain more data than atoms in the universe, ie you would need more atoms than are in the universe to create such a quantity of memory.

Now this is not say you might not have other parts of a computer that operate in a 256bit way, ie some processor may make use of 128bit or 256 bit capability, but the memory addressing space will likely never have a need to go beyond 64bit, considering the amount of memory that already makes available.

read this article about memory addressing

being that I dont want to jinx myself here like the 640k memory limit design flaw, it might be worth stating that if we can use spintronics to make a single atom contain say 8 bits of data, it might come one day to 128 bit, but I still think its a very long way off.

really 64bit is a kind of plateaux.