Colapse the stack. If you are shoore that you have finished some model turn everething into edit mesh or edit poly, exept meshsmooth or turbosmooth modifier (and then, within this modifiers turn that only in render they subdivide mesh).
Offcourse, first open a new scene, and then merge models, and colapse them, one by one.
Be carefull, only God have unlimited amount of poly's.
Great job, by the way.