External renderer, 4 threads. And please, please run a 64 bit OS.
If you use only 2 threads, one core gets to render the upper half of the scene, the other one gets the lower half. If the upper half is far more complex to render (e.g. transmapped hair + raytraced reflections + soft raytraced shadows in the right upper corner) than the lower half, the second core will sit idle once the lower half has finished rendering.
If you use 4 threads, the surface will be divided into 4 parts. Once a part has been rendered, the core that had been rendering that part will devote all its attention to another part. Same image as before (complexity in the right upper part, less complexity in the other parts) - one core will probably be dedicated to rendering the right upper part, while the other core will render the other 3 parts, resulting in a shorter total render time.

It would have been more intelligent to assign by bucket - once a core/thread completes a bucket, it'll get a fresh one to render - that would be a far more efficient use of multicore CPUs. But alas, that's not the way Firefly is built.

Maybe something for Poser 8....

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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