> currently there are 17000 but will be up to 30000 they will only be taking once lesson at the moment, possibly more later.
> I will probably use a VPS with 4gb of dedicated ram. Is that enough ...
The only formula around is the "general rule of thumb is that Moodle can support 50 concurrent usersfor every 1GB of RAM, ..." See http://docs.moodle.org/en/Installing_Moodle#Hardware
Also read http://moodle.org/mod/forum/discuss.php?d=88924 and other similar threads in this forum - use the forum-search facility.
The best starting point is the http://docs.moodle.org/en/Performance documentation.
I don't understand why the server has to deliver the final load from day one. Migrating a Moodle server is no big deal.
Hardware and performance
Some basic info needed for using Moodle
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