One workaround for complex scenes that many people seem to overlook is to simply render in layers. If you render, say, a foreground figure, midground figures and background props individually, you maintain much more control and don't choke your machine. Often I find that the foreground element benefits from AO, where the background looks just fine using P6's P4 render engine. Another benefit of this technique is it provides more flexibility for post work, where you can tweak each layer individually. You might also explore P6's Shadow Catcher, which does NOT require raytracing and works great with the P4 engine.