The R5 was validated with up to 2400 via XMP.  So you could get a matched set of 2 x 8 GB (or 4 x 4 GB) DDR4 2400 XMP aftermarket ram and should be fine, or from Dell and it will be fine.  Generally, everything else being equal, you're less likely to encounter issues with 2 matched sticks as opposed to 4.  You could do one stick, of 16GB, but then you'd lose out on dual channel mode.  At 2666, some people have had success with 2666, others have not, so it depends.  Anything beyond that, I'd say there's nothing that can be done to get your current motherboard to support a faster clock speed. 
Hi everyone, I recently wanted to upgrade the Ram speed my alienware PC that I bought from the Nov 2016 to make it to run faster. I had tried to look up multiple videos and article about how the whole process. However I got stuck on the step on the BIOS, which I couldn't enable overlock feature to change my RAM from 2133Mhz to 2400Mhz, since the overlock option on my BIOS is grey out and not able to select it. So I would like to ask if anyone know how to enable the overlock feature to select either XMP 1 and 2 in this situation? And also I would like to provide some basic information below to help if any of the information is able to determine whether my PC are able to upgrade the RAM speed or not. I appreciate for any help or suggestion.  BIOS Version: 1.0.11 Product Name: Alienware Aurora R5 Motherboard: Alienware 01NYPT (100 Series / C230 Series) CPU Type: Intel (R) Core (TM) i7-6700 CPU [NAME].40 GHz CPU Speed: 3400 MHz System Memory: 8192 MB (DDR4) Memory Speed: 2133Mhz Installed RAM: 8GB System Type: 64-bit operating system, x64-based processor [IMAGE]
Aurora R5, 2133MHz RAM, overclock issues