I jut got a 5820 which has the original 2 x 8gb modules. What would happen if I added a single compatible 16gb module (same manufacturer, same timings, same model number with one digit changed). I'm curious about the consequences. Obviously two more identical 8gb modules would be best as that makes the most of quad channel RAM. But I happen to have this 16gb module.

Did you ever figure out what configuration to use? I have the same exact issue.

Memory configuration Dell Precision 5820 Tower
Rhefernce the user manual:
Page 59 (picture of system board (Mother board) components
Table on page (?) shows memory configuration and population for 5920 systems.

OK-- after opening up the machine and removing the air shroud:
looking from the" bottom" of the picture on page 59. I.e., in the direction of air flow from the vent fans (the air shroud has an air flow direction arrow embossed on the cover).
The DIMM slots are (Left to 6, and 5 & 7 (but my memoryof this may err).
I used this to successfully install memory upgrade 4 - 32GB RDIMMs + 4 - 16GB RDIMMs (192 GB registered ECC RDIMMs) and computer recognized it A-OK Hooah!

By & By, you don't have to break the bank purchasing memory if you shop around and avoid the outrageous over-priced mamory that the computer manufacturers charge. The memory I have is lifetime guarranteed and I have used my source for memory on my Mac's (since a Mac IICi thats maby 20+ years ago), DELL (desktop & laptop) & HP computers, and HP & XEROX printers.

Take care,

@jgwam Please share suggested source. Thanks!

Data Memory Systems (DMS)

DMS stands by their warantee. Examples:

I had a 8 Gb registered ECC DIMM fail after 2+ years use on  MacPro v1,1.  DMS replaced no charge (including mailing) and no questions asked.  All they request is you return the defective memory to them.  Same for a 16 GB ECC DIM on a Dell work station.  Received the replacement on less than a week.

These were the only memory failures I have experienced in around 20 years of using DMS memory upgrades on 2 NAS devices, 9 different Computers (Mac & PC desktop and mobile work stations and laptops). and 2 printers (HP & Xerox)..

I am trying to figure out whether I can user the 4x16gb config and add two 8gb as well? The manual seems to indicate that you cant mix memory sizes. My guidance - bear in mind I am a novice.

It is related to initial RAM discovery by CPU and BIOS... arbitrary placement may work or not. But usually there are no problems with populating RAM slots on DELL workstation. Sometimes, you can install TWICE more RAM than the manual tells (depending on CPU and BIOS version). But it will never work with clocks faster than the CPU officially supports (no O.C. on Dell).

Nonuniform memory placement may disable some interleaving schemes. RAM with different timings (including extended set) WILL lead to system instability. I actively recommend not installing different RAM modules.

There is a Slight Difference ~Between W-2100 series & W-2200 series  -  In  CLOCKING.. some what &

Mhz  :  2666Mhz  vs  2933Mhz  - but all Memory Manufactures have Switched to 3200Mhz . .

U May still Find - 2666Mhz or 2933Mz  at slightly Cheaper Price .. vs  3200Mhz

I'm  Not Positive about   2Rx8  vs  1Rx8  on Your  T5820 vs what some Memory Folks

call T5820XL - which in My Book is a Intel i7 or i9 CPU . .

I have Found that 2666Mhz is Cheaper to Buy Currently , but this will change as Inventory Runs Out ./.

My Main Point is Check You 2R vs 1R  First  then Which Every is Better Buy for U  . .

2R may Run in My 1R Required T5820 W-2200 series , but It Run  Hotter & Slower . .

Where 1R is Direct Link 1 Way Full Speed Up and Full Speed back on Matching Stick . .

Memory Manager Chip If it see 2R - will want to Communicate Up & Back on same stick

This could Cut MHz in Half  per Each Stick in Actual CPU Utilization   . .

2R Memory is Cheaper  and so Is 1Rx4 which is More Like Server Ram ..

That said some Memory Retailers will call 1Rx8 Sever Ram , just Because it is Rdimms  . .

ECC Rdimms are Not same as Plain Non-Reg ECC , but UDimms are Mostly Server Ram ..

Yes this  gets  Confusing Especially When " NON- Dell Retailers "  What to Hide Info and

Play Games with Wording in Their Sales Ads !

All Memory @ Higher Mhz  will  Default  Back to  CPU & BIO's  Mhz  Rating for Your Mother Broad . .

I Bought 3200Mhz  is Runs at 2933Mhz - I did this for few Reasons - which may be Too Deep for Your Question . .  

Always Go with Hardware ~ 1/3/5/7  ~  2/4/6/8