I'm considering buy a Macbook. This is gonna be my first Mac ever, switching from PC. I'm wondering if I decide to buy one from Amazon, and then buy bigger RAM and bigger HDD to upgrade, would this void Apple's Warranty? I watched a tutorial on Youtube on how to change the RAM and it looked easy. Is it also easy to upgrade the HDD? Another question ... 120 GB 5400 rpm vs 120 GB 7200 rpm .. big difference?