The best way to look at the general health of the DB is to use the [url http://www.ora00600.com/articles/oracle-awr.html]AWR utility. From that you can get an indication as to what is happening in the DB for the time period you select for the report. It also allows you to drill down to the SQL statements.
Once you get an overall picture of how the database is performing and where there is potential room for improvement then you know where to start in testing changes.
Hope that helps,