I'm going to upgrade my RAM in my Macbook. I know that it requires DDR2 SO DIMM 667MHz 200 pin memory, but I have a few questions: 1)When it says PC 5300, does that always mean that it's 667mhz? 2)Does it make a big difference if mix RAM from two different brands? 3)Does it make a big difference if the sizes are different (in my case, it would be a 2GB stick with a 512MB stick)? Thanks.