Performance tuning of Postgres server

I have Postgres 9.1 installed on Windows server 2008 R2 machine having 32 GB RAM. For performance tuning i have to update the shared buffer to 8 GB. But after updating the shared buffer to 8 GB in postgresql.conf file i am not able to start the service. its giving error message "The Postgresql-9.1 - Postgresql server 9.1 serviceon local computer started and then stopped. Some servcies stop automatically if they are not in use by other servcies or programs".

Any one can suggest how can i update the shared buffer. Thanks in advance...