Vinay,

your initial problem may have been that not enough queries are using indexes instead of a setting too low for DB_BLOCK_BUFFERS. These buffers are used to store index blocks in memory so they can be reused by other processes, and thus avoiding so many trips to disk.

you might also try turning on tracing for a few sessions (using DBMS_SYSTEM.SET_SQL_TRACE_IN_SESSION) and looking at the trace file for the explain plans.

-John