There is no unbuffered registered memory. Typically one would refer to 2 memory types for an R710: - DDR3 unbuffered ECC memory - DDR3 registered ECC memory From the screenshot you have registered memory (also referred to as RDIMM), so you can only add more RDIMMs.
I am planning on buying new RAMs for my Dell Poweredge R710 Server . How to check the ram I am using is buffered registered or Unbuffered registered RAM?  Does this means that I have two buffered Registered ECC RAM installed on my server? Please have a look at the attachment: Server Menu screenshot 
How to check what type of RAM am I currently using?