This message was posted on behalf of Intel Corporation
Memory population rules on S3420GP is kind of complicated.
If you're using a Xeon 3400 series processor, you can use either RDIMM (registered) or UDIMM (unbuffered), but you can't mix RDIMM and UDIMM. If you're using Core i3-500 series or Pentium G6950 processor, only UDIMM is supported. ECC is required for both RDIMM and UDIMM.
Now for RDIMM you can populate all six slots. For UDIMM you can use only four DIMMs, two each channel. # of DIMMs per channel will also affect the DIMM speed. Maximum memory capacity is 32GB RDIMM or 16GB UDIMM. For more details, see the S3420GP Technical product specification, section 3.2.
Remember that you can always select memory from the Tested memory list.