What are the considerations in choosing a memory?

What works for me and others is - next time you get a new computer, get as much memory as you can afford both RAM and Hard Disk. That way any software you have will run as fast as possible
Types of Ram Minimum required ram for motherboard vis-a-vis the size of the HDD and the applications that needs to run on it. Server type of computer needs more RAM to accommodate file serving activities. Applications such as Visual and Graphical in nature require more than command-line type of application. Data transfer speed is also necessary, as well as the technology used in the development of the RAM, Disk, and any storage media.

eg: Harddisk RPM (Revolutions per minute). Same for DVD, CD-ROM, and TAPES.

RAM Data transfer rate in Megahertz. Same for Semiconductor Storage Drives(SSD)

Storage size of the memory is also needed (Physical or Logical Size).

The way the memory is managed by the computer (architecture and organization)
Speed, capacity, latency, form factor, heat/voltage, pair matching, error correction, MTBF, manufacturer/assembler
speed

compatability

maximum memory allowed
size shape model