I tracxked a thread that has an answer by dekk on the 730x memory issue
Dell states "Version 1.0.5[edit] The last BIOS released from Dell was version 1.0.5. This version has a number of limitations, most notable:
Memory SPD tRFC setting for DDR3 SDRAM is limited to 100. This limits the use of memory to 2 GB DIMM sticks.[6][16]
Limited eXtreme Memory Profile support for Profile 1 and very limited Profile 2 memory profiles.[6]
Limited overclocking support. PCH Voltages do not change, though being set in the bios.
Limited failed overclocking recovery. Have to reset the BIOS often via motherboard jumper.
A number of users have installed 12 GB (3 x 4GB DIMM sticks) with this bios version. However, while the BIOS will register the full amount of ram, memory errors will be generated when using 70% or more of physical RAM because of the tRFC limitation of 100.[6] DDR3 SDRAM memory sticks of 4 GB in size requires at least an tRFC setting of 122 or higher.[16]"
but tons of users report using 12GB on the system without an issues. on bios 1.05 and 1.06
you could get 12GB 3x4gb and if it does not work return it to the store. incompatible with system
suggested ram set was :
https://www.newegg.com/product/product.aspx?item=n82e16820148522