Serializable Isolation level for Stateless Session Bean Method

[Veena Rani]

Hi,
What will be the effect of setting the isolation level of a session bean method as serializable with oracle database? Does this also guarantee that
any databse read/update in that method will be serializable if multiple clients are calling the bean method in the clustered environment?
Will this have any effect on the performance of the other transactions being performed/
We are using WebSphere in a clustered environment with Oracle 8 database.
Our requirement is to read and then update a row in the table depending on some condition. read and update process should be exclusive, no other transaction should be able to read that row while first tranasction is reading and updating the row.
please suggest the possible approaches for this.
Regards,
Veena