What type on memory can I use?  

How can I determine what type of memory sticks my system will accept? I currently have 16GB of DDR4 2666mhz and wish to upgrade to 32GB. What does the "2666mhz" denote and what is its purpose? Thanks.

Crucial also have a memory finder. Here's their page for the Dell XPS 8930 in your specs.

Memory RAM & SSD Upgrades | | | XPS 8930 | Crucial UK

So that's telling me I can only use 2666mhz memory, is that correct?

XPS 8930 specs: https://www.dell.com/support/manuals...d5a&lang=en-us

I presume that if it has 16GB, that'd be two 8 GB DIMMs (Dual inline memory modules, aka RAM sticks).

I suggest downloading and running CPU-Z. (CPU-Z | Softwares | CPUID) If you get the .zip version, it's portable; nothing needs to be installed to use it.

If you look at the SPD tab for each slot, you'll get the exact part number for the memory DIMM. Don't worry about the frequency. CPU-Z reports the actual clock frequency, rather than the DDR (Double Data Rate effective transfer frequency). At least, it does that on my home assembled PC.

If you are able to obtain exactly the same DIMMs, that might be a good idea. It's not essential.

Companies like Crucial aren't always the cheapest, but they gurantee compatibility. I don't know whether that extends to mixing their RAM with that of other makers.

I'm sure other companies are just as reliable but I've always used the Crucial tool to buy memory for my systems and those of friends (and clients when I was working) and never had a module that didn't work as expected.