Cluster unfortunately has several meanings especially in the PostgreSQL world.
You want a stand-by server using replication.
This is possible with 8.4, see here:
http://www.postgresql.org/docs/8.4/s...ilability.html
But things got a lot easier with 9.0 and 9.1. Using streaming replication minimizes the possible loss of data in case the server goes down.
http://www.postgresql.org/docs/9.1/s...ilability.html
A good overview of all your possibilities is in the Postgres Wiki:
http://wiki.postgresql.org/wiki/Repl...ection_Pooling
I would recommend you upgrade to 9.x and use streaming replication with a hot or warm standby server.
Whether or not you need synchronous replication is something you need to decide. Synchronous means you will never lose any data, but the downside is that it has a performance impact on the main server.
If you start implementing that, I strongly recommend to subscribe to the Postgres mailing lists (-general and -admin)
You'll get excellent help there (directly from the developers!)