Use layers man, lots of em, for the different areas. remember to render with alphas so that it makes it a lot easier to composite into a full scene, that should take away a bit load on your PC.
It might also be best to render using render scripts with a sepperate scene for each part of the scene that way you can just have maya render ing without maya actually being on mental ray will use all the system resource.
"No pressure, no diamonds" Thomas Carlyle