[Paulo Mateus] "I'm using a MacPro (8 Core with 2 Gb Ram), and when I enable the multiprocessing option in After Effects (version 8.0.0.247), it just renders "normally", like I never enabled the Multiprocessing option. It doesn't render multiple frames simultaneously."
Each core in your system has to be able to address RAM for your Image Cache. Check your Memory & Cache settings and make sure that you have the Image Cache setting low enough to allow the 2GB to be split across multiple processors.
Note that you probably don't want to lower this value too much, so you should definitely make your next upgrade more RAM. You have 8 cores, each of which can use up to 3GB of RAM (2GB would be great too), you should try to upgrade to at least 8GB of RAM to take more advantage of your multiple cores.
Here are some good websites for memory:
http://www.datamem.com
http://www.macsales.com Darby EdelenDesignerLeft Coast DigitalSanta Cruz, CA