Pavil,  The R820 supports both standard and low voltage dimms. That includes the 1866MHz range as well. We offer a 32GB 1866MHz 1.5 volt dimm, part # JGGRT. As well as 16GB dimms at the 1866MHz range (part # 12C23), which appear to be standard voltage (1.5v) as well.  I hope this helps. 
Hello, I would like to ask if it is possible to add 16gb 1866MT/s memory to an R820.On Dell site it clearly states that the server accepts up to 1866MT/s yet on every single paper that i have read it states up to 1600MT/s. The target is at 512GB. And i was thinking of buying 16GB 2r4 PC3-14900R dimms. would it work or do they have to be low voltage. Maybe 32GB sticks? Well the server has 4X E5 4650 v2 cpu and a newer motherboard. So the question that remains is can you load 32 rdimm 1.5v or do they need to be 1.35v?
Dell R820 Memory upgrade