Quote from ArchAngel:
The computer definitions of the prefixes used for RAM and disk space are specific only to those uses and the deviation stems from the early days when you wouldn't have a 1,000 bit memory bank - it had to be 1024 because the # of bits had to be a power of 2 due to the design - so they warped Kilo to mean 1024 in that case and it cascaded from there.
The normal definitions he posted are completely correct and the accepted definitions for everything else - e.g., a Kilometer is 1000 meters, not 1024 meters.