PS: for php sites that only run on one server I think Xcache is also
better
then memcached, no context switching at all...
PPS: do we use an opcode cache already for the php site(s)?
      

eaccelerator?
  

xcache etc? it can be a huge improvement
      

APC is used for a PHP opcode cache, so yes, we do have this in place.
    

APC does not only store opcodes, but e.g. Mediawiki uses it to store
Objects in RAM. So memcached is not needed here.