Memory Issues

I've come very far with the bazaar scene. At a certain point a few months ago, I decided to bring in all of my geometry from Zbrush as decimated geometry which renders quickly but adds tens of thousands of polygons to the master 3DS Max file. Sure enough, I maxed out my all of my RAM and can no longer add to the scene. So, as a result, I now am running tests to find the best method of dealing with the problem. The question here is: do I want to use bump maps on low poly geometry or displacement maps. I don't like working with displacement maps alone because they are a pain to generate and take a long time to render. Here's a test render with high lighting and shadow quality settings.