2GBx4 or 4GB x 2 RAM? Which one is better? Buying full 16GB RAM now on the new iMac is just insanely expensive, I would like to upgrade to 16GB when the RAM price drop. My question is, if I buy the 4GB x 2 RAM, and later on buy another 4GB x 2 RAM of a different brand, will that slow down the speed or cause crashes? Please let me know.