I'm also running APC.
Memcached is not the same thing as an opcode cache. It directly caches database queries, more like mysql's query cache.
I installed the package from the debian archive with 'sudo aptitude install memcached.' I kept all the default settings in /etc/memcached.conf. If you have buckets of memory to spare you might want to consider raising the value for -m, but otherwise I'd leave everything as is. I added the following to conf_global.php in the IPB root directory.
$INFO['use_memcache'] = 1;
$INFO['memcache_server_1'] = "127.0.0.1";
$INFO[memcache_port_1'] = "11211";