Which Ram memory brand is compatible with alienware m15x

Many people think memory is like a fine wine, where one is better than another. As long as you get the correctly spec'ed modules there is little difference across most.

The areas you may want to look at are the thickness and the size & number of the memory chips on the module and the modules circuit board size.

Newer designs use smaller (thiner) chips. And these chips are often doubled up chips so you need 1/2 as many chips. These modules tend to run cooler (use less power) and fit into tighter spaces.

But these newer designs often cost more. Your system was designed with the older larger modules. So you don't have to worry about the fit as in other thin systems.

Here's one source that offers a cheaper option Hynix 4GB PC3-10600 DDR3 1333MHz non-ECC Unbuffered CL9 204-Pin SoDimm HMT351S6CFR8C-H9 each costs about $20 apiece.