replying to add that I have this exact problem as well.   I never pinpointed the memory limit, so that helps. 

I generally would hide/remove/change the objects in the scene until it would render.   Usually turning off texture filtering would do the trick - but there was a risk of trans-mapped hair looking awful.

Also, there were times I could kill poser, restart and be able to render the scene, so I usually give that a try.   As you say, you can tell right away if it's not going to work - shows background color, poser window min/maxes quickly etc.