Quote - I was surprised that the manual said that a duo core processor will also realize gains by using four threads. It also recommends checking Separate Processes for complex scenes, but doesn't make much effort to define what a complex scene is.

I think how complex the scene need to be varies with how much memory your machine has.

My older machine (2meg) used to crash fairly often without render as a seperate process but my 4 meg laptop works either way.