You should probably get the exact models you currently have. For example if you already have 2 sticks of 4gb Gskill 1600 cas 9 model 113 then you should get 2 more sticks of 4gb Gskill 1600 cas 9 model 113 unless your motherboard doesn't support 4 sticks. Those models/stats are for example only, the point is that you want the exact same thing for it to run optimally. Usually what you want to avoid is using different models of ram and using odd numbers of sticks. So don't mix different speeds/manufacture's of ram and don't use 3 sticks.
You should probably get the exact models you currently have. For example if you already have 2 sticks of 4gb Gskill 1600 cas 9 model 113 then you should get 2 more sticks of 4gb Gskill 1600 cas 9 model 113 unless your motherboard doesn't support 4 sticks. Those models/stats are for example only, the point is that you want the exact same thing for it to run optimally. Usually what you want to avoid is using different models of ram and using odd numbers of sticks. So don't mix different speeds/manufacture's of ram and don't use 3 sticks.