Hardware and performance

> I saw that number of users should = 50 * GB of RAM

Probably you mean http://docs.moodle.org/en/Installing_Moodle#How_many_users.3F

But also check this http://docs.moodle.org/en/Installing_Moodle#Requirements
"memory_limit should be at least 16M (32M is recommended for Moodle 1.7 and 40M for Moodle 1.8 or later). Large sites may need more than 128M. PHP 5.2.x requires higher memory_limit values than previous versions of PHP. 64bit operating systems require even more memory."