Clustering, mirroriing, or replication?

We're trying to figure out how we can implement a reasonably simple cluster
of postgres servers on a private network at our store locations. The idea
is to have a group of 2-6 PCs each be able to share data in a replicated
manner. Having a separate database server is not an option due to the
immense cost and logistics (we have over 800 stores), so we need to find a
way to mirror or replicate data between all the nodes currently up and on
the network. Some nodes might be not always be up, so we need a failover
system. I'm testing to see if the dbmirror that comes with Postgres will
work, but I haven't got it completely figured out yet.

If anyone has experience setting up and maintaing clusters with
replication/mirroring, I'd like to hear of your experience and maybe pick
your brain a little. Any help you could give would be excellent!

Thanks!

Jon Brisbin
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Development Programmer
NPC International
(620) 235-7726