I bumped up from 4GB to 8GB yesterday. Nice!
It's not so much that GCIII runs faster (doubtful). It's that the whole system accommodates GCIII much more easily. I can actually run Firefox at the same time! And switch to the desktop without praying to St. Jude.
Watching the Perfomance Monitor, GCIII takes 6GB all by its own self. Load a game and it takes another 0.7GB - less than I imagined.