2400 is native speed of Z270 chipset of R6.  When XMP is enabled ram is overclocked.  Dell validated up to 2667 but they did not push for the real ceiling (?).  You may or may not be able to run 64 GB at overclock freq depending on BIOS default setting and ram spec you get.  Just bear in mind that the higher frequency of overclocked ram, the lower stability of your system (i.e., crash, BSOD blue screen).  The theoretical ceiling of Z270 ram is 4133.  Dell BIOS tends to be more restrictive for user to play around and void warranty.

Thank you very much for your answer!

I don't really need the extra speed from overclocking and try the 2666 ( as they are in Stock ) if the speed causes problems I will try 2400 and stay with the original speed.

Thanks a lot!

Best

Dob

The aurora R6 is especially picky and this is really a YMMV scenario.  I've seen users report the R6 running at 2666 with XMP enabled, only running with XMP disabled, and not posting with the retail RAM. Personally I have 4 sticks running at 64gb with xmp enabled using the retail (amazon) hyperx fury in my Aurora. 

Don't forget to buy a matched set for all four stick for best results, if you must mix sets only install each set in the same colored tabs (white or black).  If it doesn't work, you can try updating to the latest BIOS.

See here for one example:

https://www.dell.com/community/Alienware-General-Read-Only/Is-this-RAM-compatible-with-my-PC-Aurora-R6/td-p/5519164

Do note that the retail hyperx fury has different timings than the dell hyperx fury.  The service manual is referring to the Dell hyperx fury which must be purchased from Dell.  That is the only validated hyperx fury.  Of course other RAM may work fine, including the retail hyperx, it's just not validated.  The link you have is the 2019 version of the hyperx, which has an updated heat spreader and better timings then the prior version hyperx fury.  So it also looks different ftom what would have come stock with the R6 or have been purchased from Dell.

I think it matters if you have an i7-7700 or i7-7700k processor. At least with the i7-7700 (not K) you can't use the BIOs to overclock. The manual you get with the computer only notes you can't use XMP at 64GB (See the Setup and Specifications manual https://www.dell.com/support/manuals/en-us/alienware-aurora-r6-desktop/alienware-aurora-r6-setupandspecs/set-up-your-computer?guid=guid-02a836d6-932b-4f7c-8bfa-0643923149d0&lang=en-us ).  But, the Service Manual indicates XMP (and thus 2666) only works with two 8GB for 16GB. Service Manual - https://www.dell.com/support/manuals/en-us/alienware-aurora-r6-desktop/alienware-aurora-r6-sm/before-working-inside-your-computer?guid=guid-5d3b1051-9384-409a-8d5b-9b53bd496de8&lang=en-us . I don't know that the difference between 2400 and 2666 is that noticeable. So, you can get XMP and 2666 with 16GB. But, only that.