Once Support Assist identifies my XPS 8940 desktop, is the "Parts & Upgrades" list for MY build, or all 8940s?

Sorry, I'm confused...! 

You said: "I want to upgrade my RAM to 32 GB", and then you said current RAM is "...two DIMM, 16GB, 3200, 1RX8,16,DDR4, NU cards".  Do you mean you have 2x 8 GB DIMMs for a total of 16 GB now?  

If you have 2x 8GB now, you could just install another 2x 8 GB in the two empty slots for a total of 32 GB.  You always want to install RAM in matched pairs, so it runs in "dual channel mode" that's faster than "single channel model" which is how RAM will run when you install only a single module (eg, 1x 32 GB) or don't have patched pairs.

And you don't have to buy RAM from Dell.  Check out crucial.com for RAM compatible with the XPS 8940., and at better prices. If they recommend specific modules for your PC model, they guarantee it works or they take it back...

The parts and upgrades list from your screenshot is for all builds of XPS 8940.  They are matched up with the main, expansion list from here  https://www.dell.com/en-us/shop/pfydresults/269537?categoryId=8490

Good advise from Ron, it's cheaper to acquire compatible memory from third party.  Use the memory configurations below to find the most feasible upgrade plan for which you can use your current memory in combine with new memory in supported configurations below.

• 8 GB, 2 x 4 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel
• 8 GB, 1 x 8 GB, DDR4, 2666 MHz/2933 MHz, UDIMM
• 16 GB, 2 x 8 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel
• 16 GB, 1 x 16 GB, DDR4, 2666 MHz/2933 MHz, UDIMM
• 32 GB, 1 x 32 GB, DDR4, 2666 MHz/2933 MHz, UDIMM
• 32 GB, 2 x 16 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel
• 64 GB, 2 x 32 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel
• 64 GB, 4 x 16 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel
• 128 GB, 4 x 32 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel

NOTE: The computer is shipped with 3200 MHz memory. However, with Intel Core i3/i5 processors, it is limited to 2666 MHz. With Intel Core i7/i9 processors, it is limited to 2933 MHz.  Same rule applies to third party memory, higher speed memory will be downclocked accordingly.

@RoHe​ Sorry, should've said I currently have 32 (as two 16s) and I want to add ANOTHER 32 to equal 64. So you're saying rather than add one 32GB card, I should add two more 16GB cards like the "32 GB, 2 x 16 GB, DDR4, 2666 MHz/2933 MHz, UDIMM, dual-channel" option Chino lists above? 

Yes, add 2x16 GB.  I still wouldn't buy it from Dell.

Crucial has a kit for XPS 8940 with 2x 16 GB of DDR4-3200 • CL=22 • NON-ECC • UDIMM • 288-pin • 1.2V • 2Rx8/1Rx8 • PC4-25600 for ~$80. Compare that price with Dell's...

@RoHe​ Thanks! I'll try Crucial. Though I've seen a few people say they had problems. But maybe they did something else wrong. 

Is there a chance that adding the 32GB RAM will exceed the 2933 MHz limit and end up not making much of a difference? Not sure how that works. 

Not aware of any major problems with Crucial RAM, aside from an occasional bad module, but that doesn't mean it couldn't happen.

If you buy RAM that's on Crucial's list of compatible RAM for XPS 8940, they'll take it back if it doesn't work, or replace it if it's just a bum module, at no cost to you. So you're protected.  I'd get the 2x 16 GB kit that's listed for XPS 8940 rather than 2 individual 16 GB modules so you're more likely to get a well-matched pair.

You never mentioned why you want to increase RAM to 64 GB. Are you running very RAM-intensive software, eg Photoshop, video editing, massive spreadsheets, have dozens of browser windows open at same time, etc..?  If not, you're probably not going to see much/any performance improvements.

Don't understand "Is there a chance that adding the 32GB RAM will exceed the 2933 MHz limit and end up not making much of a difference? Not sure how that works. "

XPS 8940 can use up to 128 GB of RAM, so adding 32 GB to the 32 GB you have now is below that limit. Both the Dell OEM and Crucial RAM have rated speeds of 3200 MHz, but the i7 CPU in your XPS 8940 can only run RAM at up to 2933 MHz. So 3200 MHz RAM just clocks down to 2933 MHz.

After you install the new RAM, you can check its actual running speed using CPU-Z (free). Keep in mind you have to multiply the RAM speed shown in CPU-Z by 2 because it's DDR RAM.  So CPU-Z will report something ~1460 MHz when the RAM is running at ~2933 MHz.

And while in CPU-Z, confirm that Channel # says either "Dual" or "2x64-bit", meaning RAM is running in dual channel mode.

@RoHe​ I do run several Adobe programs at once, so that's why I'm upgrading.

I have no knowledge about processors, so I didn't know if the 2933 MHz limit meant I'm limited as to how much RAM can be added before it hits that limit. 

Thanks for the CPU-Z tip! I'll give it a try with my fingers crossed. 

Maximum RAM limits and max RAM speed limits are totally different.  So no need to worry. You should be fine with 64 GB of 3200 MHz RAM.

Post back and let us know how the upgrade goes...

@RoHe​ Upgrade went without a hitch! I'm loving the extra power and not having to do regular restarts when working on my photos!

@strux​  - Well done!

Did you get the new 32 GB of RAM from Crucial or somewhere else?

Enjoy the improved performance....