Browsing users is the same thing : users active in 5 minutes period. Moodle doc indicates 1Gb RAM is necessary for 50 concurrent users (~= active users / 5)
I just ran a few tests on the database and yes, we will add more RAM, 32Gb should be enough. 150k users makes the user table alone go up to 10Gb.
We will use moodle 2.0, and maybe change mySQL for postgres, which seems way better at handling large tables.