Thanks for the quick reply Josh. Do you know if I use dual-rank RDIMMs if I can add memory to slots A7, A8, and A9 as well?

That configuration looks correct. From the user guide starting on page 130 here are the memory requirements ftp.dell.com/.../poweredge-r710_Owner%27s%20Manual_en-us.pdf

Quad ranked dimms 2 per channel.

A1,A2,A3,A4,A5,A6 for the slots.

Your configuration meets both of those requirements so it should work fine.

I may be wrong, but if you fully populate, you need to be aware memory speed will drop back to 800.

Isn't that what I said - "if you FULLY populate"?

No, you're not wrong ...  page 130 of the manual:

For single or dual-rank memory modules:
• One memory module per channel supports up to 1333 MHz.
• Two memory modules per channel supports up to 1067 MHz.
• Three memory modules per channel are limited to 800 MHz,
regardless of memory module speed.

When you said "I may be wrong", it left some doubt as to how sure you were about it ... so, I showed you where and what the manual it says, so you can be more sure next time. ;)