I have a mid 2009 Macbook Pro 5,5. It has 4G of memory now and I'd like to upgrade to 8G. Where is the best place to buy memory? I've looked around and have seen prices ranging from $50 - $100. I don't know how to judge memory quality. Would the cheaper memory work as well as the more expensive? Should I buy local so I can easily return it if there is a problem or is an internet purchase ok?