The Moodle docs information is a good start. It is certainly a lot better than doing nothing. There loads of information online about tuning Apache (e.g. http://www.tecmint.com/apache-performance-tuning/).
I think it's always important to monitor you system to understand what's going on. Simplest is probably munin - especially if you enable Apache's ExtendedStatus setting. That will tell you over time, how much resource Apache (and other things) are using.