The computer has 48GB memory (or memories?)

It would have to be 'memory', since computer memory is measurable, not countable. In this particular case, 'memories' would be confusing as well as mistaken, since it would imply that the computer has several distinct memories, kept apart for some reason, and either each is 48GB capacity, or they total 48GB. In my experience, the usual phrase is '48 GB of memory' , to avoid this problem.

It is

The computer has 48GB of memory.

Memories are the contents of one's memory, but I've never heard about contents of computer memory being called memories.

That'd be 48 gigabytes of RAM, Random-Access Memory, not Memories.

I don't like it, but it looks like some vendors (Apple and HP) have started dropping Random Access:

http://www.apple.com/why-mac/compare/notebooks.html http://www.shopping.hp.com/laptop

"2GB or 4GB memory" "6GB memory"

Dell still spells it out for you:

http://www.dell.com/us/p/inspiron-15r-combo-mod/pd?oc=fndor05&model_id=inspiron-15r-combo-mod

Memory 3GB3 Dual Channel DDR3 SDRAM at 1333MHz