I have a late 2012 iMac 27"with an 3.4 GHz Intel Core i7 processor, I've been getting message that I don't have enough memory and to force quit apps. The iMac has the standard 8GB fitted 2X4GB Modules. My question is should I get another 2X4GB modules or do I just add 1X8GB Module, not sure if it makes any difference 2X4GB Modules compared to 1 8GB