Getting things humming on a 4 gig server

[Ian Evans]

Will soon be migrating from a 2 gig CentOS server to a 4 Gig Ubuntu. 
We'll be running nginx (of course), php-fpm, the fastcgi cache, MariaDB 
and Zend Opcache.

Though I realize that 4 gigs of RAM is still modest, it is twice what 
we've previously had. I'm just curious for some general suggestions (or 
pointers to articles) as how to allocate 4 gigs between php-fpm, the 
opcache, the fastcgi cache and DB caches.

Would appreciate any tips or pointers.