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Buyer's Guide: MacBook Pro Memory
See Checking memory use on Mac OS X to understand how memory is used.
The 2011 MacBook Pro accepts up to 16GB memory, which can make a large difference in performance for some users. The 2009 and 1010 MacBook and MacBook Pro are limited to 8GB memory.
For most users, 8GB is enough, and for photo users, 8GB provides a nice bump up in performance over 4GB.
Heavy-duty users can go to 12GB or 16GB.