Hi, I have Kingston DDR3 1600MHz 8GB Hyperx Blu (like this http://www.amazon.com/Kingston-1600MHz-240-pin-KHX1600C10D3B1-8G/dp/B007TTEHUY ), and I'd like to upgrade my RAM by 4 GB so it goes up to 12GB; however, I've heard that if I can only stack RAMs of the same model and not different ones. Is there a 4GB RAM that is compatible with my current 8GB one, or do I really have to buy the same one?