When mixing ram you really need the same speed, timing, and voltage. I have found that when you start mixing ram with different timings and voltage you have problems. Why not buy the same as you already have.
It is sold out right now and I could only do 1 rebate per household thing. I guess I will wait for something to come along at the same timing and voltage than. I can live on 4 gigs for awhile. I just wanted to find another 4 gigs for some good overkill. thanks.