Your 2666 RAM will run at 2400.

Yes. The motherboard might default to 2400 MHz (or even 2133), and you’ll need to enable XMP to push it higher—but even then, the CPU’s memory controller maxes out at 2400. So effectively, you’re buying 2666 MHz sticks that will clock down to 2400 MHz automatically.

That’s fine. Your i7-7700K won’t see any benefit from the extra 266 MHz, and you’re not losing performance by running it at the CPU’s limit. Just make sure you enable XMP in BIOS to get to 2400 instead of the base JEDEC speed (likely 2133). If you skip that step, you’ll leave performance on the table.

You’re not wasting money, either—2666 is usually the same price as 2400 these days. If it costs more, buy 2400 instead.

Future You won’t notice the difference.