The best is what you are already running preferrably from the same manufacturer's batch. Whenever adding memory you want to match as close as possible if not the same as the memory already in use. You would simply order another pair of the same Corsair memory to insure seeing the best results from the same type and brand there.
I have Black, yellow, black, yellow. So my question was can i get something better then 667 or should i match the 667 or can i bump it up ? Whats the fastest ram that will fit into those extra slots.The new Asus board here allows for 1+3 or 2+4 fpr a two dimm setup. Many 939 boards will specify having a pair of dimms in the blue or sometimes green colored slots while the black colored are the secondary for each channel. Cromewell's advice on referring to the manual included with the board is always the first thing done here and is the thing to do there as well.