Concurrency question

I have a table of unique numbers, call them PINs. Once a PIN has been assigned to a user, it needs to be unavailable to other users.

I have a query which identifies unallocated PINs. However, I don't know the best way to avoid the situation where two transactions could grab the same PIN, if a second transaction began before the first transaction committed.

Is there a convention/best practice for handling this scenario? I'm aware of SERIALIZABLE, but am concerned about the potential impact on throughput.

Oracle 8.1.7.3.0 and 9.2.0.3.0 on Solaris 8. Client application is Tomcat 4.1 and java servlets.

Pointers appreciated.

-- 
John Bossert

In what concerns you much, do not think that you
have companions: know that you are alone in the world.

             -- Henry David Thoreau