Never mix ram. At worst it doesn't work at best your ram runs in mixed mode with a hefty performance penalty. Only use the exact same make and model ram (cl, cmd rate, capacity, speed must all be identical) and even that won't guarantee that the new stick will play well with your old one. The only sure fire way to make sure they play nice is to buy a dual channel kit and replace all your ram. I personally try the same make and model first...if it fails I return it and buy a whole new kit, just like I am advising you to do now.