I didn't catch how big your actual site is but you can usually provision your RAM based on the number of Apache workers and whether or not the DB is running on the same server. Moodle.org for example runs 700 Apache workers and both the main webserver and database servers have 16GB RAM. To contrast, 300 workers ran relatively ok on just 7GB RAM. The beauty of VM is that you can adjust this pretty much on the fly with minimal downtime.
Matt
Hardware and performance
Slow disk reads with Linux VMs, Moodle 1.9
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