Hi There,
I have basically the same system as you. I could be wrong about this, but as far as I know each core uses 1 gig when multiprocessing. So you need to adjust your percentage of use to a number that gets you less that 1 g. Mine is set to %30.
BUT, I have found that for simple renders it is much better to keep multiprocessing off. It takes longer just for it to fire up all the cores than the initial render would have taken in the first place.
Good luck,
Joey