here are the steps for tuning. they are listed in the order. If possiable do #1 then #2 etc. It will do you no good to increase db_block_buffers if some user is doing a full table scan on a table with millions of rows and Gigs of data. (unless of course you use 64bit oracle and have 64GB of RAM ... )

1. Tune App design
2. Tune the App. Tune all the SQL in the App.
3. Tune O/S configureation (kernel changes, etc)
4. Tune Memory Structures (Buffer cache, row cache, lib cache, log buffer ...)
5. Tune Disk I/O usage
6. Detect and eliminate Resource Contention (Latch in buffers, dispatchers...)


because you have an existing application you cannot do much about the design (#1). You can look at the SQL that is ececuting and find the bad ones.
this is straight out of the book.

Andrew
OCP