I have a DB similar to what you have. My DB has 10-15 million inserts a day and I did the following to get performance.

1. Parallel inserts (Increases the use of multiple CPUs)
2. Set LOG_CHECKPOINT_INTERVAL to a huge setting, thus turning it off
3. Set LOG_CHECKPOINT_TIMEOUT to 0, thus turning it off
4. PCT_USED to 0 (This gets rid of freelists)
5. Redo Logs on RAID 0 mount points

I would greatly increase your DB_Block_Buffers and Log_buffers. Watch out for redo latch contention and redo log I/O contention.

I like to use OEM, "iostat -xnc 3", "vmstat 3" and top to diagnos my problems.