Hi I have a mb with 4 ddr3 ram slots, two of which are filled by 4gb ram sticks, I am looking to get more ram but I do not want o spend too much because this is not a priority upgrade. will I be able to get one more stick of ram to put into one of the empty slots or will i have to get 2? and if i get 2 will they have to be the same as my other ram? thank you for your time