Thanks for the quick answer, I'll give it a try with this setup and an updated version of PHP.
Basically, I didn't bother and I only installed the yum epel (I'm under CentOS) version of PHP... Apart from opcache, what modules would you recommand? I currently have the ones mentioned here.
I've also tried installing eaccelerator as recommanded in the doc but under this version of PHP it seems quite buggy: is this the reasn why you're using opcache instead?