I've installed APC on my server and the load is cached very good with it, no problems so far. The number of concurrent users is good < 90 but after 100 it gets shaky.
Htop shows a constant use %100 of all cores after 100 users.
The DB seems to be the culprit here, I've attached a screenshot.
Take a look at the PHP Info here: http://kilifvekapak.com/info.php