Read the XPS 8940 FAQ here: https://www.dell.com/community/XPS-Desktops/FAQ-XPS-8940/m-p/7675308#M53201

Also, see the Setup and Specifications. The speed your RAM runs at depends upon the processor, so if you have an i7 or i9 it will run at 2933 MHz, but it comes with 3200 MHz RAM.

Note that 48 GB of memory is not listed as one of the memory configurations that is supported. I would purchase a single 1 x 16 GB 3200 MHz memory module to match and pair with the memory module that ships with your computer. That way your 32 GB will run in dual-channel mode. I recommend purchasing memory from crucial.com, they guarantee compatibility. I think 32 GB of memory is plenty but of course, it depends upon what type of work you do on your computer. 

Install your memory in slots with similar color securing clips, which makes them operate in pairs.

I have 2 x 8GB 2933GHz (i9) onboard my new XPS 8940. There are 4 slots available. 

I don't want to ditch the 2 x 8GB which came with it, but want to expand memory..

Can I leave the 2 x 8 in place & add 2 x 32GB = 80 GB total ?

Or leave 2 x 8GB in place & add 2 x 16GB = 48GB total ?

Any recommendations for where to buy the memory ? 

@banchang  - Just keep in mind that Dell hasn't validated either of those two RAM configs in the XPS 8940, but that doesn't mean they won't work.

Is this a 10th or 11th Gen i9 CPU?

The PC came with 3200 MHz RAM, so you should go with similar speed RAM for the upgrade. It will run at 2933 MHz with a 10th Gen CPU, or at 3200 MHz with an 11th Gen CPU.

Many users around here buy RAM for Dell PCs at crucial.com because they guarantee it works, when they recommended it for a specific PC model, or they take it back. But it looks like crucial is only offering 32 GB-2666 MHz RAM for the XPS 8940 right now. And that RAM will slow all installed RAM to 2666 MHz.

And FYI, RAM is in very short supply everywhere these days and prices are high...

Mine is 11th Gen Intel(R) Core(TM) i9-11900K processor (8-Core, 16M Cache, 3.5GHz to 5.3GHz

The onboard 2x8GB is 2933 GHz.

Don’t they recommend, if expanding, to go with same speed ?

So not 3200 GHz but go with 2933 GHz consistent with the existing onboard memory ? 

@banchang  So you definitely want to go with 3200 MHz RAM for max performance.

According to the specs, Dell supplies the XPS 8940 with 3200 GHz RAM, but the specs also say the 3200 MHz RAM can only run at 2933 MHz when there's 10th Gen i7 and i9 CPUs.

11th Gen Intel CPUs are supposed to run 3200 GHz RAM at that fully rated speed.

So if you mix RAM with different rated speeds, it will only run at the speed of the slowest.

Mixing ram is not supported by DELL or INTEL.

Vendors use different CAS Speed, Rank, Voltage, etc.

ADATA doesn't mix with Micron, Hynix, Samsung etc.

So you need to run CPU-Z and find out which one is your Ram

C5N22 16GB DDR4, 3200MHz, 2Gx64, 2RX8, Unbuffered, 288 pin, Dual Rank, 1.2V, Non-ECC, Non-Encrypted, 

Adata AO2P32NCST2-BZ7SHD,

Hynix HMA82GU6DJR8N-XN,

Kingston KC5N22-HYD/KC5N22-MIE,

Micron MTA16ATF2G64AZ-3G2J1,

Samsung M378A2K43EB1-CWE

Slowest speed working mixed is wishful thinking. 

Unstable  or not working at all is more likely.

These are not the same across the board which is why even from dell there are various part numbers for Ram Sticks.  They do not all come from the same place or have matched parameters.

Standard black chassis USA sales page

SE silver chassis USA sales page

Drivers

Manuals

CPU
R56MP Intel i9-10900KF Comet Lake-S, 3.70GHz, 20MB, 125w, 10 core
3PH75 Intel i9-10900K Comet Lake-S, 3.70GHz, 20MB, 125w, 10 core
48X0T Intel i9-10900F Comet Lake-S, 2.80GHz, 20MB, 65w, 10 core
35G7Y Intel i9-10900 Comet Lake-S, 2.80GHz, 20MB, 65w, 10 core
NJV41 Intel i7-10700KF Comet Lake-S, 3.80GHz, 16MB, 125w, 8 core
C2T71 Intel i7-10700K Comet Lake-S, 3.80GHz, 16MB, 125w, 8 core
JCPGM Intel i7-10700F Comet Lake-S, 2.90GHz, 16MB, 65w, 8 core
DX5NG Intel i7-10700 Comet Lake-S, 2.90GHz, 16MB, 65w, 8 core
FGJ4W Intel i5-10600KF Comet Lake-S, 4.10GHz, 12MB, 125w, 6 core
R1PRN Intel i5-10600K Comet Lake-S, 4.10GHz, 12MB, 125w, 6 core
2D8P4 Intel i5-10400F Comet Lake-S, 2.90GHz, 12MB, 65w, 6 core
25PHV Intel i5-10400 Comet Lake-S, 2.90GHz, 12MB, 65w, 6 core
CT26X Intel i3-10100 Comet Lake-S, 3.60GHz, 8MB, 65w, 4 core

PSU
2VD0G 360w Liteon
5K7J8 500w Delta
99TPH 500w Liteon
Y7R0X 500w Chicony

Ram
Four UDIMM slots
Maximum 128GB
2666 MHz, i3/i5 CPU
2933 MHz, i7/i9 CPU

128GB = 4x32GB
64GB = 2x32GB, 4x16GB
32GB = 1x32GB, 2x16GB
16GB = 1x16GB, 2x8GB
8GB = 1x8GB, 2x4GB

To be clear from a Dell warranty and policy perspective, a user should not expect our OEM BIOS and OEM tested/validated memory sticks to work without issues using non-tested/non-validated retail memory sticks.

The XPS 8940 is shipped with 3200MHz memory. However, with Intel Core i3/i5 processors, it is limited to 2666MHz. With Intel Core i7/i9 processors, it is limited to 2933MHz.

732YD 32GB DDR4, 3200MHz, 2Rx8, Unbuffered, 288 pin, Dual Rank, 1.2V, Non-ECC, Non-Encrypted, Hynix HMAA4GU6AJR8N-XN/HMAA4GU6CJR8N-XN, Micron MTA16ATF4G64AZ-3G2B1/MTA16ATF4G64AZ-3G2E1, Samsung M378A4G43AB2-CWE

C5N22 16GB DDR4, 3200MHz, 2Gx64, 2RX8, Unbuffered, 288 pin, Dual Rank, 1.2V, Non-ECC, Non-Encrypted, Adata AO2P32NCST2-BZ7SHD, Hynix HMA82GU6DJR8N-XN, Kingston KC5N22-HYD/KC5N22-MIE, Micron MTA16ATF2G64AZ-3G2J1, Samsung M378A2K43EB1-CWE

V0M5R 8GB DDR4, 3200MHz, 1Gx64, 1RX8, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted, Adata AO2P32NC8T1-BZ5SHD, Hynix HMA81GU6CJR8N-XN/HMA81GU6DJR8N-XN, Kingston KV0M5R-HYD/KV0M5R-MIE, Micron MTA8ATF1G64AZ-3G2J1, Samsung M378A1K43EB2-CWE

36FTX 4GB DDR4, 3200MHz, 512x64, 1RX16, Unbuffered, 288 pin, Single Rank, 1.2V, Non-ECC, Non-Encrypted, Adata AO2P32NC4U1-BZNSHD, Hynix HMA851U6DJR6N-XN, Kingston K36FTX-MIE, Micron MTA4ATF51264AZ-3G2J1, Samsung M378A5244CB0-CWE

Video cards
GPCKW MSI GeForce RTX 2070 Super, 8GB, Three DP 1.4/HDMI 2.0b
G7CH1 MSI GeForce RTX 2060 Super, 8GB, DP 1.4/HDMI 2.0b/DVI-D
DHRKF MSI GeForce RTX 2060, 6GB, DP 1.4/HDMI 2.0b/DVI-D
KPNXF MSI GeForce GTX 1660 Ti, 6GB, DP 1.4/HDMI 2.0b/DVI-D
7MKYT MSI GeForce GTX 1660 Super, 6GB, DP 1.4/HDMI 2.0b/DVI-D
4WY5P ECS GeForce GTX 1650 Super, 4GB, DP 1.4/HDMI 2.0b/DVI-D
7HNWN ECS GeForce GT 1030, 2GB, HDMI 2.0/DVI-D
4FCCX ECS RX 5700 XT, 8GB, Three DP 1.4/HDMI 2.0b

38MPY ECS RX 5700, 8GB, Three DP 1.4/HDMI 2.0b
NH5PX MSI RX 5600 XT, 6GB, Three DP 1.4/HDMI 2.0

5V7N7 MSI RX 5300, 3GB, Two DP 1.4/HDMI 2.0b
Onboard Intel UHD 630, Two DP 1.4/HDMI 2.0