Howtoforge posted a guide about setting up memcached and the PHP5 memcache module on Debian Etch

This guide explains how to install memcached and the PHP5 memcache module on a Debian Etch system with Apache2. memcached is a daemon that can store objects in the system's memory (e.g. results of database queries) which can speed up your web site tremendously. You can use memcached over a network (i.e., install your web application on one server and memcached on another server), but usually you install both on one server to avoid the networking overhead.

Installing memcached And The PHP5 memcache Module On Debian Etch (Apache2)