m15 R1, maximum RAM

Hi @luca_puggelli  thank you for Alienware m15 R1 clarification. Welcome to this user to user discussion forum. 

The CPU limits the RAM capacity. so check CPU specification first.

Dell Support for Alienware m15 Memory says max Memory per slot is 16 GB, max Configuration supported 32 GB DDR4 at 2666 MHz (2 x 16 GB). 

If you want guaranteed compatibility and do not want to buy direct from Dell/Alienware, go to Crucial. Crucial is the retail arm of Micron, and Micron is one of the few companies that actually make DRAM. This means that you are getting it straight from the source, rather than a re-packaged product which is what many other companies sell. You can see the guaranteed parts for the Alienware m15 here: Memory RAM & SSD Upgrades | alienware | alienware notebooks | Alienware m15 | Crucial.com, but this only reiterates what Dell Support says. 

Providing you get the same type of RAM, purchase from Crucial has the added benefit of their excellent returns policy. So trying a matched pair of 64 GB DDR4 at 2666 MHz (2 x 32 GB) can be attempted. There is no hazard trying 64 GB of RAM, but expect only 32 GB to be acknowledged based on what RAM the CPU will support. 

You can do 64Gb but the way to do it is by putting one stick in and loading the OS, Then shut it down and put the other stick in because thats how they got 128GB working on the AREA 51m.