Well that's strange if you can render it in Poser 7 but not Poser 8, with separate process checked.
But in your first post you said Poser Pro, not Poser 7. Is that 64 bit Poser Pro? Because that would explain why it renders there and not in Poser 8.

At any rate, even in Poser 8 in a separate process, I think it would still choke on a scene using more than 2 GB of RAM.