How to upgrade MacBook Pro RAM to 4GB?

I've been looking around on sites like newegg because I'd like to upgrade my macbook pro. I bought it about 8 months ago and it came with 2gb but it can handle 4gb (i'm pretty sure).

Do I have to get a 4gb to replace it or can I just buy 2gb to add on? And where can I get it for cheap?

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Tags: Mac, MacBook, RAM