Ag:
If you are getting the memory warning it means that you don't have enough RAM to see benefits from MP. As a matter of fact you might even slow your renders down since each process requires it's own allotment of RAM to render and they will be memory starved if there isn't enough and it will cause the system to memory swap which is very very slow.
There is some good news however. If you upgrade to 8.0.2 (released yesterday) there is now a system pref you can set to tell AE how many procs you want to use (in previous versions it used all the procs every time) for MP.
This is from the 8.0.2 tech notes:
Multiprocessing performance issues with multi-core machines, especially high-end 8-core hardware. When all 8 cores try to render a complex or memory intensive composition with Render Mutliple Frames Simultaneously, the application can become memory starved if each process has insufficient memory. To remedy this, there is a text preference to control the maximum number of cores that Render Multiple Frames Simultaneously will use. Open the text preference file and look in the ["MP"] section for: "MaxNumberOfProcesses" = "0". "0" is the default setting, which disables this preference setting. For better performance on an 8-core machine with 8GB RAM, for example, change the "0" setting to "4" and save the preference file and restart After Effects. This restricts multiprocessing with RenderMultiple Frames Simultaneously to 4 cores, each getting approximately 2GB of RAM.
I plan on updating the BG Renderer to include an option to set this pref, but unfortunately it sets it for the whole app, not the BG render process.
-Lloyd
