I saw you mentioning a Mac Pro. How many cores does it have? If it's rendering in more threads than Poser 7 (which is limited to 4), then it will inevitably require more memory for rendering, as each rendering thread requires its own chunk of memory for keeping the bucket buffer, micropolygons, shader state etc. You can try if it works when rendering using less threads.

At what point does it crash? Before the render, during or at the end?