It all depends on what kind of composition you’re rendering. There’s no one answer, even for a given system, since the demands of each project are different (or even the demands of each phase of project).
If you’re working with standard-definition-sized compositions and footage items, then you could probably benefit from enabling the Render Multiple Frames Simultaneously multiprocessing feature.
If you’re working with bigger compositions and footage items, you don’t really have enough RAM to use the Render Multiple Frames Simultaneously multiprocessing feature. For HD work, you want at least 2GB per process, and then the OS and other software take their bite…
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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