XPS 8910, RAM upgrade compatibility (speed, channel, and rank)

It's DDR4, 2133 MHz, non-ECC, UDIMM. max 64 GB.

Dell memory specs and DIMM slots configurations  https://www.dell.com/support/manuals/en-us/xps-8910-desktop/xps-8910-desktop/memory?guid=guid-7a8b9be6-87bc-4cd9-8232-897fb250ed5a&lang=en-us

Compatible memory upgrade from Crucial  https://www.crucial.com/compatible-upgrade-for/dell/xps-8910

Memory specs from Intel with controller, bandwidth, channel for Skylake processors  https://www.intel.com/content/www/us/en/products/sku/88184/intel-core-i56500-processor-6m-cache-up-to-3-60-ghz/specifications.html

2400 or 2666 MHz RAM depending on the price; there's no point in anything faster.  It is a dual-channel board so single modules or matched pairs perform best.

There's no point running high performance RAM in this system -- either it will be wasted money, or if it's XMP RAM, you may find it's actually slower (it'll default to 2133 since the system does not support XMP) than 2400 or 2666 MHz RAM.

@ejn63​ 2666 seems fully functional so I'll buy 2666 but xmp is is slower than both 2400 and 2666 right?

So 2133 is equally relevant speed to 2666 and they're faster than XMP right?

Any RAM you install faster than 2133 will simply downclock to 2133, so it makes no difference.