"Microprocessors are generally defined by the number of
pieces of information they can manipulate at once. A 32-bit microprocessor,
for example, can move, add, subtract or multiply numbers that are 32
zeros and ones long. A 64-bit processor is capable of handling binary
numbers consisting of a string of 64 zeros and ones.
It is the length of the information string that determines the amount of
random-access memory, or RAM, a processor can make use of during a task.
The more RAM per task, the greater the computing power.
Thirty-two bit processors are limited to a theoretical maximum of handling
4 billion bytes of RAM per task. (A byte equals eight bits.) But by the
magic of exponential math, a 64-bit processor can theoretically handle
16 quintillion (or 16 billion billion) bytes of RAM. For all current practical
purposes, that is an infinite amount."
from the New York Times