First Here is the link to the server information page. http://www.intel.com/p/en_US/support/highlights/server/s3420gp
This is the link to the tested memory list.http://serverconfigurator.intel.com/configure-memory.aspx?id=MzQwMSMxLDMzOTgjMSwzNDI3IzE=&ShowNavigation=false
These are dimms that Intel has tested with the board and confirmed proper operation. (What you are refering to as Intel Certified)
Other DIMMS may work fine, but Intel does not test or provide support for the system configuration using untested dimms so it is best to use tested memory.
The TPS (technical Product Specification) is the complete specification for all things http://download.intel.com/support/motherboards/server/s3420gp/sb/s3420gp_tps_r2_4.pdf
Page 18 & 19 covers the possiable types, size , ranking and number of DIMMs
Register DIMMS (RDIMM) supports up to 32G total memory using quad rank, 2g technology dimms (4 - 8G dimms)
UDIMMS only support up to 16 G (4- 4G Udimms) See tables 5 & 7 pg 23 & 24
Slot population rules vary depending on the type of memory and the type processor in use.
Memory speed will also vary depending on configuration so it is best to study the chart to see how to optimize with your CPU.
Okay thanks for your answer.
First i wanted to buy two of this sets, but what i understand from your story is that this are UDIMMS and that i can't install more than two of this sets onto the motherboard (16GB) although i still have 2 free slots. According to the Intel list i do can put three of this kits on this mobo (all slots are taken, 24GB installed). Is this right?