"Simultaneous" users is a peculiar problem if you are running large laboratories and are giving an introduction to freshmen who have no idea what to do. In that case, you instruct them step-by-step and they ALL hit the same button at the same time. In my experience, that load is equivalent to a 20,000 user school where 1000 users are online and all doing different tasks.
Are hosting companies chucking off users for using Moodle? On the contrary, many hosting companies are aggressively courting Moodle users. Netmondo.com recently offered a discount to Moodle users (they currently cost US$29 per year for a site with Moodle installed running unlimited MySQL databases, 500MB space) I have seen providers with rules that forbid running any chat programs on their servers. Those kind of providers cater to people with static web sites, and of course a Moodle user or any eLearning site user, or interactive site creator would not qualify.
How about the competitors? Do WebCT and Blackboard make more or less demands on servers than Moodle? I would be curious to hear if any one has experience in this area.
And how do you calculate hardware needs for a school? Martyn reports his school has 1600 users. I would guess to calculate the size of server you would need to include the number of courses, the number of times a student would access per week, the types of activities (chat is usually the most demanding), the internal transfer rates on campus and other important points.