I learned that innodb_buffer_pool_size makes innodb behave like an in memory db. If I give innodb_buffer_pool_size enough ram for the entire db could memcached improve further the performance, or memcached would be unnecessary in such a setup?
The InnoDB Buffer Pool is the home of three classes of informational structures
If you make the InnoDB Buffer Pool large enough, then all data you have accessed will essentially stay in RAM. Any data you have never read but finally decided to read will then have data and indexes cached in the InnoDB Buffer Pool. Please read my post : What are the main differences between InnoDB and MyISAM? on how to determine the size of a good InnoDB Buffer Pool.
Given a large enough InnoDB Buffer Pool, I would rule out using memcached.