i need to upgrade my MacBook's RAM, but don't want to pay the $200 from the apple store. i found this on walmart's website:http://www.walmart.com/catalog/product.do?product_id=9855015, and i'm pretty sure it will work on my MacBook, but not sure. i was also looking at this one: http://www.microcenter.com/single_product_results.phtml?product_id=0256690, i know this will work on a mac, but its more expensive as well... which should i get? will the one from walmart work? thanks.