[Dave]

I believe APC is developed by the contributors of PHP and is officially supported(?), that would be a good option to choose. I don't think there's a huge difference between the PHP caching modules.

You can do aggressive complete page caching with Varnish, but you must configure it so it will only cache content with users who are not logged in (check if cookie set for example). Users who are logged in obviously get shown different phrases and things on their screen.

Using MariaDB instead of MySQL will also show a nice improvement, MariaDB also has very nice caching features.
It's a matter of installing MariaDB and moving over your db files, no need to change any PHP settings since MariaDB works with all of the existing MySQL modules.