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I am looking at a combo on NewEgg, https://www.newegg.com/Product/ComboBundleDetails.aspx?ItemList=Combo.3579279, that has the following pieces:

  • Intel Core i7-7700K Kaby Lake Quad-Core 4.2 GHz

  • ASRock Z270 KILLER SLI/AC MB

  • CORSAIR Vengeance LPX 16GB DDR4 3000

  • Phanteks Eclipse P400 Series Case

  • EVGA 700 B1 100-B1-0700-K1 700W PSU

  • 250GB SSD, CM H100i V2 Cooler

I am a bit confused because the i7 and the ASRock motherboard both specify DDR4 2400 RAM.

Looking at other questions I found a link to the Crucial memory and one of the suggested alternatives is Ballistix Tactical 4GB DDR4-2666 UDIMM.

Is the speed, such as DDR4 2400 just a suggestion and faster can be used or what?

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    I'll add an answer in an hour or so when I'm more available, but basically, RAM faster than 2400 is "overclocked" using XMP out of the box. – JMY1000 Jan 3 '18 at 23:31
  • @JMY1000 thank you. I am looking forward to an answer. I took a look at the XMP article but did not understand much beyond there is data about the memory stick encoded in the stick and which is used at boot time as part of configuring the hardware run time environment. My primary concern is whether using the faster RAM will be a problem. My secondary question is whether using the faster RAM will be an asset. As I know nothing about over clocking and tweaking settings, I would hope that at boot the processor and memory and motherboard can get together and decide how to run as fast as possible. – Richard Chambers Jan 4 '18 at 3:47
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TL;DR: It will run at that speed, and it will be an asset in some workloads.

Memory overclocking and XMP

Intel specifies that their CPUs (here, the 7700K) are guaranteed to work with DDR4-2133/2400, DDR3L-1333/1600 @ 1.35V. Anything beyond that isn't officially part of their specifications, or guaranteed to work. However, in practice, it will.

What's going on here is overclocking–basically, pushing stuff beyond it's manufacturer rated speeds. This practice isn't unsafe, and, in the case of memory, is basically supported by the manufacturer. Here, the RAM manufacturer certifies that the RAM will work at 2666 MHz. Though the motherboard manufacturer only guarantees that RAM at 2133/2400 MHz will work, it supports overclocking up to 3733 MHz and beyond (and effectively guarantees it will work.) Similarly, Intel allows their CPUs to work with overclocked RAM (and effectively guarantees it will work.)

Because memory overclocking takes work, Intel created something called XMP, or Extreme Memory Profile. Basically, this is a list of settings stored on the memory modules that tell the motherboard what speeds it says it can overclock to and what timings it needs to do so. Most motherboards will then automatically discover and load those settings, though some may require manual configuration. The end result is the same though: the memory runs at its "overclocked" speed.

RAM speed as an asset

Though I won't dive into it too much, how much–if at all–RAM speed matters depends on your workload. This video does a much better job examining it then I could ever do, but the basic conclusion is that it matters a little in some games (more on Ryzen then other platforms) and varies widely for other workloads. If you're building for gaming, don't sweat it too much; if you're building for some other task, Google around and see what other people have experienced.

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