Memory or brand can be an issue in many Dell's. If you buy memory, get at a place that has a liberal return policy.  Dell's have had a history or memory compatibility with non-Dell brand memory.   Over the years we have suggested Crucial memory as it is one brand that is guaranteed to work in a Dell.  

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/xps-8900

Mixing brands/type can be a disaster too, and not just in a Dell.  I have a self built system using an ASUS ROG motherboard with an i7 6700K CPU.  I have 16GB of Corsair Dominator and 16GB of GSkill that is comparable to the Corsair.  Either the Corsair or GSkill work OK but if I put both in, it won't boot.

I found one source of info - with a working example:

XPS 8900 Memory Upgrade to 64 GB 

To achieve 64 GBs of RAM, you would have to use 4x sticks of 16 Gb DDR4-2133.

If you use DDR4-2666, only two populated slots will be recognized - for a total of 32 Gb.

Yes, 2666 is technically backwards compatible, and the BIOS will clock it down to 2133, but it won't recognize all 4 sticks for some reason.

Sorry I don't have sources handy, but that's the conclusion I've have reached after much reading of other Dell community forum posts.

Here's a second source, again notice that mixing in DDR4-2666 causes complications in a 4 slot arrangement:

XPS 8900, won't accept 64GB RAM, only up to 56GB 

Hi, fireberd,

You seem quite knowledgeable; I hope you don’t mind if I pick your brain for a minute…or two…

I presently have an XPS 8900 with SK Hynix 16GB (2 X 8GB) of DDR4-2133 / PC4-17000 DDR4 SDRAM UDIMM but want to add another 16GB...

My question is this... Amazon offers "Crucial RAM 32GB Kit (2x16GB) DDR4 2400 MHz CL17 Desktop Memory CT2K16G4DFD824A" would that work in my XPS 8900…? I assume the 2400 is backwards compatible but would it work with the SK Hynix memory that Dell originally sold me with the system…?

I look forward to your feedback.

Thank you

@Dinah-Moe-Humm I am not sure fireberd would see your question since you did not add @fireberd in your post.

The Amazon kit is 32 GB but you stated you wanted to add another 16 GB. If you look at crucial.com, the 32 GB kit is compatible and they also list a 16 GB kit (CT2K8G4DFS824A).

In the majority of cases we see here over the years mixing brands of memory, although the same electronic specifications, results in problems.  This is not just a "Dell only" issue; I had a desktop with an ASUS gamer motherboard and had some GSkill memory and some Corsair memory both the same speed and other specs.  If I installed only the Corsair or only the GSkill it worked OK but if I installed both the POST would detect a memory error and would not boot.

@Vic384 @fireberd So I am 100% clear...If I purchase the Crucial 32GB Amazon kit (CT2K16G4DFD824A) which is DDR4-2400 and not DDR4-2133 it will still work...Correct...?

@borntodeal @fireberd is correct that sometimes mixing brands of memory will result in problems. If you really want to purchase the Crucial memory, I would check the price at crucial.com. By using crucial.com's memory tool to find compatible memory they guarantee 100% compatibility They have a 45-day no-hassle money-back guarantee.