Understand that if you go with quad ranked memory you can only populate 2 slots per channel as the limit of ranks per channel on these servers is 8.

Example:

16GB PC3L 4Rx4 10600R

• only 2 sticks per channel for a total of 8 ranks (populating the 3rd channel will likely result in a memory configuration error on boot or even a no boot situation)

16GB PC3L 2Rx4 10600R

• all 3 channels can be populated for a total of 6 ranks

For the 11th gen Dell servers if it supports triple channel memory, each channel you populate will drop the speed of the memory. For example using one channel with 10600 memory will yield the full 1333Mhz but populating two channels will yield 1066Mhz. Populating all 3 channels will drop the speed to 800Mhz.

You may think that quad rank memory would run at a higher clock speed right? Nope... turns out it runs at 1066Mhz even when one channel is populated (see source below).

Now is it faster though? I'm not sure. Consensus seems to be that more ranks might be used for higher capacity modules? If that is true it wouldn't matter since though the 610 and 710 servers can run quad rank memory, they can't take advantage of DIMMs more than 16GB anyway. It seems as though quad rank modules *are* cheaper than dual rank modules (going by listings I've found on Ebay) so it could be a consideration...but even if they are cheaper, for the price you could get more density at the same speed using 8GB modules in a memory optimized configuration and be able to take advantage of the triple DIMM configuration as well as the higher RAM capacity overall (96GB vs 64GB). The only thing I can think of is you could save a bit of power if you are going for memory optimized configuration and don't need 96GB? Or maybe if someone already has quad rank DIMMs on hand already...

The 11th gen servers only support 2GB UDIMM modules up to 12 modules so you can have up to 24GB of UDIMM memory.

If you have a 5600 CPU it appears you can use the "low" voltage RAM that operates at 1.35V instead of 1.5V which could yield a power savings...though in a home lab where you may only be using one or two servers the power savings might be negligible (source below:"Low voltage DIMM power savings")...but I guess it is something if you're upgrading anyway. Either way it is nice to know that the support for low voltage DIMMs is there. 

It appears that if you are wanting to upgrade your RAM capacity, figure out what you want and go with the highest density modules that fit in 3 dimms per channel per CPU for best performance.

For example if you want 48GB of RAM, you could populate all 3 slots on 2 channels across the dual CPUs with 4GB sticks of  2Rx4 10600R but your server will run at 1066Mhz memory speed where as if you populated one channel with 8 GB sticks you'll be able to take advantage of the full 1333Mhz speed supported by 5600 Xeons.

Is it important? Does it matter? probably not...but since these poor dinosaurs of a by-gone age of Westmere goodness are already at a disadvantage, why not squeeze the max performance out while they're still worth plugging into the wall?

If you already have the DIMMs you need to reach your RAM capacity it might not be worth spending a bunch of cash upgrading RAM just to get the higher clock speed...or it could be I guess it depends on your use case...but if you're ready to upgrade your RAM, It is probably worth considering upgrading density instead of quantity.

(Second link is archived version on forum just in case the PDF disappears from Dell's site)

Dell link: https://www.dell.com/downloads/global/products/pedge/en/server-poweredge-r710-tech-guidebook.pdf 

Archived link: server-poweredge-r710-tech-guidebook.pdf 

Low voltage DIMM power savings: 

Dataram's article on 1.35V vs 1.5V DIMMs: http://www.dataram.com/blog/?p=102

Tom's shareware testing of HyperX LoVo: https://www.tomshardware.com/reviews/lovo-ddr3-power,2650.html