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My Macbook Pro came with 4GB memory but has a capacity of 8GB. If I want to upgrade to 8GB, can I just buy a 4GB memory stick and add that in, or will I have to buy two 4GB memory sticks? I know there are two slots in there, but don't know if only one or both are filled with memory.

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up vote 3 down vote accepted

Macbook Pros have 2x2 GB RAM for reasons of cost and performance.

For best performance, fill both memory slots and install an identical memory module in each slot.

This applies to the mid 2009 and mid 2010 models, I'm not sure about the earlier models.

Apple tell you this e.g. here for the current model:

4GB (two 2GB SO-DIMMs) of 1066MHz DDR3 memory; two SO-DIMM slots support up to 8GB

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