Can you post a screen shot of your render settings? That 600MB left is mighty close to the end if you have some hefty shadow settings. I would suggest disabling Smoothing and filtering first -- they use a lot of oomph, and then try them each one at a time to see what the impact is.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)