Getting "memory configuration is not optimal press f1 to continue" After memory is installed

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Getting "memory configuration is not optimal press f1 to continue" After memory is installed

[ Edited ]

I replaced the entire fleet of 2gb memory sticks in a Dell Poweredge R715 with: Crucial 16GB Kit (2 x 8GB) DDR3-1600 ECC RDIMM (Part#CT5321400).

I have two processors, so following the manual, I believed the correct slots for the two sticks to be A1 and B1. 

When starting/rebooting the server I am prompted with the listed memory config. not optimal message. When I look at the hardware configuration it is show the ram as being 1333.

What're the next steps in resolving this message and creating an optimal config?