Hi, I'm Jewel with Kingston Technical Support and would like to offer our assistance. The part number KHX1600C9D3K3/12GX is a kit of three 4GB modules, KHX1600C9D3K6/12GX is a kit of six 2GB modules and KHX1600C9D3K4/12GX would indicate a kit of four 3GB modules which is nonexistent. If you already have 6GB installed you probably have part number KHX1600C9D3K3/6GX which is a kit of three 2GB modules. Finding a valid 2GB modules in today's market will be difficult. Since your motherboard is a Dual Channel board I would recommend getting a kit of two 4GB modules for a total of 8GB, part number KHX1600C9D3K2/8GX. The matching single 2GB part number would be KHX1600C9AD3/2G, this was discontinued at the begining of the year. I would not recommend just adding a 2GB module to what you currently have as there is a large chance of compatibility issues since as time goes by the exact components in memory (even of the same PN) change since memory is constantly evolving. That is why it is always best to purchase memory in kits or all at the same time to make sure they are the same. For any further assistance, please call us at 1-800-435-0640 (USA and Canada only) M - F 6am - 6pm PT and I or another available Technician will assist you.
Thank you for selecting Kingston as your upgrade partner.