Now I got 2x4gb Samsung 2RX4 PC3 10600 R . Can I add one more ? What is the best (and cheap) candidate?
Yes, they should work with the new 16GB RDIMM.
I don’t have any eBay link. I bough it from China taobao.com (a place even worse than eBay), which is cheaper and easier for me (because I am in Hong Kong). But their seller can’t quite speak English, and often don’t know how to make international deal. Not recommended for overseas buyer.
P.S. the link of the DIMM now I am using is in my signature.
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Your best approach is get another two matching 4GB or maybe get 4x8GB since the X5980 is quad-channel.
Very sure OP want to say X5680 or X5690, there is no X5980 avail on the cMP. And both of them are triple channel, not quad channel.