Among many dynamic views oracle has added a new view,
V$MEMORY_TARGET_ADVICE. The purpose of this view is
to give recommendations related to the
sizing of the memory_target parameter.

Before going further please make sure that memory
management is set to automatic in your database,
otherwise this feature will not work.

Following is a example query to this view.
SQL> select * from v$memory_target_advice order by memory_size_factor;

Here the memory allocated to the database is 512,
because the memory_size_factor is set to 1.

Also note that the ESTD_DB_TIME_FACTOR is also
set to 1.

Now in the example if the database memory size is
increased by .25 that is from 512 to 640, We will
gain a slight increase in performance because
ESTD_DB_TIME will drop from 5207 to 5040.

Increasing the memory more than 640 will not yield
any performance gain, because the DB_TIME factor
will be same ex 5004 as seen from columns 4 to 6.

NOTE: The data shown above is dependent on AWR
snapshots, so make sure it is enabled.