wooaaww, you should REALLY decrease db_block_buffers !!!
I dunno how big your instance is and how many connections you have, but having a SGA size which is about the physical RAM size of the server is performance-killing ...
you can perfectly have multiple instances with any SGA size, but the important thing is that the total of the SGA sizes should not be more than about half to 60% maximum of the physical RAM ...