WAL does the same thing to DB journaling does to the FS.
Plus allows you to roll back (PITR).
As for the RAM, it will be in ram as long as OS decides to keep it in
RAM cache, and/or its in the shared buffers memory.
Unless you have a lot of doubt about the two, I don't think it makes
too much sens to setup ramdisk table space yourself. But try it, and
see yourself.
Make sure that you have logic in place, that would set it up, before
postgresql starts up, in case you'll reboot, or something.