My friend bought a Dell computer a few months ago, the XPS 8500. It came with 16GB of RAM, but because he does work on this computer he needs 16GB. He would like to save money and just get another 8GB instead of buying a whole new set of 16GB. Since it was prebuild he doesn't know the brand of memory that he has, so does he need to match the brand, model, CAS, and/or speed of the RAM?
It came wit 2x4GB of RAM so it's already in dual-channel but it has another 2 slots of another 2x4GB dual channel setup to make 4x4GB of RAM giving him 16GB of RAM. 8GB currently isn't enough for him because he uses some RAM intensive programs to do 3D editing.
Perfect, then he needs a 2x4GB Kit @1600Mhz, and put them on slots 2 and 4.
He can choose any brand he likes but is important he match the frequency of 1600, anything higher will be downclocked 1600 anyway.