you are using 64-bit windows and max ?
if not, each process is limited to 2GB. There is a 3GB switch,
that allows just that - but without 64 bit additional RAM only helps
for using multiple applications parallel - not speeding up the rendering.
8x speed increase is also very unlikely. It all depends on how
much RAM your scene needs. Your scene might only need 2GB
in the first place - more RAM would not do anything.
TB