I recently mentioned this tip in a class and since no one seemed to know it, I thought it would be worthwhile here. If you are rendering a very large scene and are getting close to running out of RAM, here’s a way that will save you a few hundred extra MBs from being consumed without slowing down rendering at all like other procedures (such as rendering in dynamic mode or with proxies).
Simply save the scene in wireframe mode with one maximized viewport and reload the scene. As soon as you change to Smooth+Highlights mode, you should see a lot of RAM consumed, with more being consumed for larger scenes. If you switch to Smooth+Highlights mode before you render, you are basically giving away that RAM, and there is no way to get it back. But if you just load a scene and immediately render without going into Smooth+Highlights first, you will have all that extra RAM to render with. Open your Task Manager and monitor your RAM consumption to verify. The larger the scene, the more RAM you save. Roughly speaking, you should expect to see a savings of 100MB for every 1million polygons your scene contains.