Taking from a original 10g installation:

• To change them you need to edit the PFILE (not possible to change with ALTER SYSTEM the ('db_cache_size=0'):

SQL> CREATE PFILE FROM SPFILE;
SQL> EXIT
$vi $ORACLE_HOME/dbs/init<SID>.ora

• Change the following values:

sga_max_size=900M
sga_target=900M
pga_aggregate_target=900M
java_soft_sessionspace_limit=0
shared_pool_reserved_size=15M

• Remove, if exist, the following paramenters

db_cache_size
java_pool_size
shared_pool_size

• Recreate a SPFILE

SQL> CREATE SPFILE FROM PFILE

• Bounce the database

• On 10g if we set SGA_TARGET!=0 then the DB_CACHE_SIZE, SHARED_POOL_SIZE, LARGE_POOL_SIZE and JAVA_POOL_SIZE would be tunned automatically. It's a better option.

• On pdblhcb I've putted SGA_MAX_SIZE=900M and SGA_TARGET=900M with good results.

Update 20.10.2005: Some of the instance parameters should be adjusted after the database installation specifically for our internal needs. These are:

processes=300

-- MiguelAnjo - 21 Mar 2005