I have a Mac Pro 5,1 2.8 single with 16GB (4x4GB). I got a great price on a 16GB stick but it wasn't playing nice with the other memory. After doing research, I guess I got a 16GB RDIMM and my other memory are UDIMMs. So much for my cheap way of upgrading to 28GB. So, do I keep the one stick of 16GB and sell the 4x4GB? There's the part of me that's afraid that RDIMMs are harder to come by and that even if I do buy more RDIMMs, they still won't play nice together. Also, how can you tell if it's an RDIMM or UDIMM if it has a heatspreader covering it?