If I order the stock standard for the iMac 4GB (2x2) and want to upgrade to 16 through OWC. Is it better to get 16GB total from OWC and try to sell the default 2 sticks of Apple ram or is it better to get 8GB of Apple Ram (2 x 4GB) and fill the rest in with OWC. Also is there any compatibility issues mixing owc ram with apple?