Are you using RDIMMs or UDIMMs?
The Memory Configuration Tool can help you choose the correct memory -http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=17625
If you use 1066 or 1333 memory, there are limitations on how many DIMMs you can populate per channel:
DIMMs populated per channel Speed Ranks per channel 1 1066, 1333 Single Rank, Dual Rank 1 1066 Quad Rank 2 1066, 1333 Single Rank, Dual Rank 2 800* Quad Rank 3 800* Single Rank, Dual Rank
The TPS has this information in section 22.214.171.124 - http://www.intel.com/support/motherboards/server/s3420gp/sb/CS-030742.htm
Thanks. I decided to go for 2 8gb dual rank 1333 to start. If I need to add, I can easily double and still run at 1333. How about the other question? How much ram is recommended for 10 users on win 2008 r2 x64 standard. I would love to have a clue for the next server project I do. I would have a baseline for users vs ram and what board I pick out. I saw some that can take 192gb ram with multiple processors.