I would like to know the performances of Memcached on remote server (on same LAN) with disk caching. Besides Memcached is a scalable cache solution, would there be any advantage of using Memcached with respect to performance when compared to disk caching.
From my personal experience I've found memcached isn't as fast as disk cache. I believe this is because of the OS's disk IO's caching, but memcached allows for a "scalable" cache, meaning if you have more than 1 server accessing the same cache data, it will scale (especially since memcached have a very low CPU overhead compared to PHP). The only way to allow more than 1 machine to access a disk cache at once is network mount which is surely going to kill the speed of the accesses. Also one other thing you have to worry about with file caching is garbage collection to prevent disk from being saturated.
As your site(s) scale, you might want to change your mind later, so whatever choice you make, use a cache wrapper, so you can easily change your method. Zend provides a good API.