The two you mentioned: using RMAN for backup and MTS. It is not required, but is recommended if you are using either of the above. If you don't use it, memory for user sessions and RMAN backup is allocated from the shared pool, which in turn may cause fewer SQL statements to be cached.

Default size is zero. Minimum size is 600K. Max size is OS dependent, but will be at least 2GB.

From Oracle8i DBA Bible by Gennick.