The forums have been online since moodle existed, good thing you did your research fully before putting question - I honestly admire your efforts.
Now, just to clear your understandings, that 50 user per GB was applicable until Moodle 1.9, it came down to 20 users per GB as Moodle 2.0 stepped in, and even lower now, as mentioned 10-20 user per GB. (I must add, if other considerations are to be made, like use of SSD Drives now in servers, then they can come up to a more realistic figure allocating higher number of users per GB)
Now this figure 10 - 20 is really dependent upon what your users will be doing, the lowest figure of 10 is for high database
(Like Chat, Quizzes) and upper limit of 20 for (Logged in users browsing site, downloading files, posting on forums etc etc), the figurue of 50 per GB may still can stand true if you have opened your Moodle for guests and they dont have to authenticate themselves to use the site.
now, assume you have users that do mix of activities (Like taking quizzes, posting on forums), then a good figure for you to consider would be 15 user per GB (median for 10-20).
BUT, that allocation of Memory is purely based on one factor, i.e: no other alteration is done to the server, means you are using 1 SATA drive with mechanical spindle, so you are actually effecting the drive's throughput there, so if you have a good overall system configured (Like RAID, and SSD Drives, and Database
on separate server/drive), then you can even allocate 30 users per GB taking quizzes without bringing down your server to its knees.