I always keep my undo levels down around 5.  I only use it occasionally because like most long time Poser users, we learned to live without it for so long.

Have you tried rendering the scene in pieces?  Background, mid ground and foreground and then compositing in your image editor?

Is there any way to you can lower texture resolutions on you mid and background figures?  Sometime getting rid of specular and even displacement and bump maps can help.