Well in a perfect world you would like all your RAM matching, right down to the size of the stick. Thats the best way to rule out an errors. however it's a very expensive way to go about things as most people will upgrade over time and you wind up with mix and match. Since you have some that are 1333 an one that is 1600 your system defaults to the slowest speed so all your RAM is running at 1333. for better performance make sure the pair of 2 gig sticks are in the same channel, If you have four slots they should be two different colors. make sure the 2 gig sticks are on the same color. If they are all the same color then make sure the 2GB ones are not directly next to each other. So first slot 4GB, second slot 2GB, 3d slot empty, fourth slot 2 GB. Its still possible for issues to happen with mixxed pieces but if you havent had any problems then luck smiles on you. If you cant get them all at the same time, try to keep them as close as you can, same brand an model an latency ect..