In the last thread on this topic, I suggested that you have 2-4 GB per core for multi-processing.
If you don't have enough RAM to spread among your multi-processing cores, multi-processing becomes slower than a single core processing. What happens is memory starvation: once the computer runs out of RAM, it swaps some RAM out to hard disk to allow one process to work, then when the next process requests RAM, it swaps something else out to disk, and so forth, until something needs a piece of the original, already-swapped RAM; then it swaps something else to load back the original memory from disk.
Having free RAM is a good thing, because it means your computer is still usable. 4GB per core is the upper limit for what you could use, and you may stay well below that.
You mentioned your comp has only 1 layer of footage, so naturally the RAM usage will not be high. Add another couple layers, or another couple hundred (my last project had around 220 layers, and that's modest by some standards -- it's amazing how quickly they add up), and you will see why I suggest so much RAM. Just to see, I have started re-rendering that project, and my RAM usage is hovering around 17-18 GB. That leaves enough free for me to have Photoshop, Illustrator, Final Cut, Mail, and a web browser open while I render.
Walter Soyka, Principal
Keen Live, Inc.
Presentation, Motion Graphics & Widescreen Design
RenderBreak: A Blog on Innovation in Production
