Yeah, I'm not sure either. I've just been studying up a lot on the ZBRUSH subject but I haven't put much into practice yet. It could be a map resolution issue. Unfortunately I'm at work right now so I can't check my 'notes'.

One thing you could try is deleting most of the geometry and just render the head with your displacement map and see how that flies. That should keep your comp from crashing. I've also seen some folks using lower rez displacement in concert with bump for faster renders. I would think you could even use the same map. That way your renders wouldn't be such RAM hogs.

Keep us posted!

Dave