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The image stabilization process within iMovie (I am assuming you are on version 09 or higher? ) is relying mostly on raw processing power more than it is memory. It utilizes both the CPU cores.

You can check to see what memory is available, as well as Processor utilization by having Actvity monitor open while you work on your project. Simply type in "Activity Monitor" in Spotlight and it should be the first application that comes up.


It will show a graph of processor usage, as well as available memory, and how much memory iMovie is using.
 
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