I'm using an M17x R3 right now. It's a great laptop.
If it already has 8GiB of memory installed I would suggest leaving it. An extra 8GiB would be nice, but the other two DIMM slots are located underneath the keyboard and are require some disassembly of the laptop.
If you would like to add more memory, I recommend Kingston HyperX, specifically the set with the part number
Get two of those for 16GiB.
Make sure that you get the /8GX model as these have the XMP memory profiles which make configuring the high speed memory easy, the /8G do not have XMP profiles.