To be clear, in your particular case, this is a server MOBO with either RDIMM or UDIMM (ECC) memory. My comments were regarding Voltage.
REF: TYAN S8236 (S8236WGM3NR) -
http://www.tyan.com/product_SKU_spec.aspx?ProductType=MB&pid=668&SKU=600000176
DIMM Type / Speed U/RDDR3 & LV RDDR3, 800/1066/1333 MHz
Capacity Up to 256GB RDIMM/ 64GB UDIMM
RDIMM = Registered (buffered) memory
UDIMM = Unbuffered memory ; I don't recommend them in servers.
RDDR3 = ECC Registered DDR3
U/ = Unbuffered (ECC in this case) ; 'U/RDDR3' is meant as ... (U) Unbuffered DDR3 or (R) Registered DDR3
LV = Low Voltage (1.35v)
Simply stated, you have (2) Types of memory with (3) Frequencies and (2) Voltage's to choose from with this server MOBO. The ideal choice is LV (1.35v) Registered DDR3 ECC RAM, and the Frequency depends on the AMD CPU but in most cases DDR3-1333. Density 4GB, 8GB, 16GB entirely depends on your requirements and size/volume of data and transactions.
Start here for memory options - http://www.tyan.com/support_download_memory.aspx?model=S.S8236