The warning message seems to happen in Maya when you don't assign the output of a ramp to a material then apply the material to the model , but as the other post says its only a warning not a fatal error .

You can assign more memory to Maya.

But i think you would be better off using render layers so your not trying to render every object in a scene at the same time you can setup several layers one for he for-ground back ground and middle section and comp them back together in post .