Looking at the ASRock Fatality X370 Gaming K4 for my 1700x and I'm checking the QVL list for the ram
http://www.asrock.com/mb/AMD/Fatal1ty%20X370%20Gaming%20K4/index.us.asp#Memory

1.
I found a compatible 2666Mhz kit (crucial BLT2C8G4D26AFTA) but the 3000 Mhz version is just ??5 more (BLT2C8G4D30AETA).

Could I buy the 3000 Mhz version and run it at 2666 guaranteed and possibly 2933 with updates or is there a big chance I will be out of luck and have to run it at 2133 Mhz since it's not on the list?

None of these kits are compatible according to the crucial website.

2.
Then there is a single stick kit from Kingston on the ASRock QVL (HX426C15FB/8) and on their website the kit (8gbx2) HX426C15FBK2/16 is listed as compatible. Is it safe to assume the kit will work?
https://www.kingston.com/us/memory/search?DeviceType=2&Mfr=ASR&Line=Motherboard&Model=96075

Thanks for your input

teril wrote: Looking at the ASRock Fatality X370 Gaming K4 for my 1700x and I'm checking the QVL list for the ram http://www.asrock.com/mb/AMD/Fatal1ty%20X370%20Gaming%20K4/index.us.asp#Memory 1. I found a compatible 2666Mhz kit (crucial BLT2C8G4D26AFTA) but the 3000 Mhz version is just ??5 more (BLT2C8G4D30AETA). Could I buy the 3000 Mhz version and run it at 2666 guaranteed and possibly 2933 with updates or is there a big chance I will be out of luck and have to run it at 2133 Mhz since it's not on the list? None of these kits are compatible according to the crucial website. 2. Then there is a single stick kit from Kingston on the ASRock QVL (HX426C15FB/8) and on their website the kit (8gbx2) HX426C15FBK2/16 is listed as compatible. Is it safe to assume the kit will work? https://www.kingston.com/us/memory/search?DeviceType=2&Mfr=ASR&Line=Motherboard&Model=96075 Thanks for your input

The 3000 Crucial model is most likely not included in the support list because Ryzen does not support a memory speed of 3000. IF the model is otherwise the same, it should work at lower speeds, such as 2933 or 2666.  But given the issues with Ryzen memory compatibility, unless I personally had that memory model running in the same board, etc, no one can 100% guarantee it will work.

Also, the XMP profiles designed for Intel systems, the only ones that use DDR4 memory before Ryzen, won't work with Ryzen in general. Sorry but just a warning, Ryzen memory compatibility is the biggest issue (and about the only one) we are dealing with.

Is the Crucial 2666 kit in your board's QVL list? Memory manufactures have been terrible in updating their compatibility lists. We know of memory that works fine with Ryzen that has zero listings of Ryzen systems in their compatibility lists.

The best I can tell you about the Kingston kits is it seems to simply be the difference between a single and dual DIMM kit, so it should work.

I'm running 8GX-D4-3200-MR 4x8GB at 2666 currently.  These are Hynix