Linxxs, Sorry you're having memory issues with the server. The first issue you are seeing , which when trying to mix the dimms together, is that the new dimms you purchased are Rdimms. The original dimms you have in the server are Udimms, which Rdimms and Udimms can't be mixed in the same system. Hence the No Post you are seeing. Now when installing just the 4GB Rdimms, the reason for the "Non-Optimal" error is that with the R715 with dual rank Rdimms, optimal would be 1333mhz dimms, where the 4GB rdimms you ordered are 1066mhz. Now these dimms will work in the server, but is not Optimal, hence the error. Just a side note when considering the options, with Udimms you are limited to just 2gb dimms, so expanding to would only allow you to add more 2gb dimms. With the rdimms you can run as is, but will have the error, or you can trade them in for 1333mhz dimms. Just some things to consider. Let me know if this helps.
Hello,  We have a bunch (35 ish) R715s all configured exactly the same. Each with dual AMD Processors and from the factory had 8 x 2gb (1Rx8 PC3L-10600E-09-10-D0) Memory modules in it. Recently I wanted to test adding extra ram. So I purchased 8 x 4gb (2Rx4 PC3-10600R-9-10-E1) ram. This is based on both Dells manuals and crucials webpage as to what this server would accept. When trying to install this ram these are scenarios and what comes up:  1) Add the 8 new modules to the empty "Black" slots. The machine won't boot at all just buzzes. 2) Take the 8 old modules out of the "White" slots and REPLACE with the 8 new modules. It boots but says "Warning: The installed memory configuration is not optimal".  Yet the bios reads 32gb of ram. 3) Take it down to only 4 modules (new modules). Putting them in as described in "A1,B1,A2,B2" slots. Same "not optimal" message appears and it reads 16gb of ram.  I have also ensured that i am running the latest firmware available. Can you please explain what i am doing wrong and my best option for the ram that I have? 
R715 Memory Configuration Not Optimal