Is memcached capable of making full use of multi-core? Or is there any way tuning this?
memcached is multi-threaded by default and has no problem saturating many cores. It's a bit harder to saturate all cores on more massively parallel boxes (e.g. a 256-core CMT box) just because it gets harder to get the data in and out of the network.
If you find areas where some sort of contention is preventing you from saturating cores, file a bug or start a discussion.