If you have sga_target set on your instance it can also be a good idea to set db_cache_size. This can especially be true when your top events are CPU and file IO. When an instance has insufficient memory, Oracle will often spend significant CPU time managing memory by moving it among various pools. Setting the db_cache_size will set the minimum size of the db_cache. Oracle can increase the size if necessary.
To set the db_cache_size to 500 MB you would use the following command:
alter system set db_cache_size=500M;