It can take up to 8 GB of RAM. If you are going to upgrade go for the 8 GB. You can find it on tigerdirect.com or newegg.com and the upgrade is a pretty simple process.
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Here is one example http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=744487&CatId=4153
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Here is another http://www.newegg.com/Product/Product.aspx?Item=N82E16820233179
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Here is the guide http://www.ifixit.com/Guide/Installing+MacBook+Pro+13-Inch+Unibody+Mid+2009+RAM/1334/1
It can take up to 8 GB of RAM. If you are going to upgrade go for the 8 GB. You can find it on tigerdirect.com or newegg.com and the upgrade is a pretty simple process.