Largest calculatable number

What is the largest number that can be held in (for example) a gigabyte of memory?

>>16
Even "random" number generators follow algorithms.
http://en.wikipedia.org/wiki/Pseudorandom_number_generator

Although, filling it with data from a natural source (something as simple as rolling a D10 <where 10 = 0>) is possible and considered random, it would take an incalculable lenght of time.