resource monitoring

I am running a few web based applications with postgres on the backend.
  We have a few app servers load balanced all connecting to a dedicated
postgres server.  As usage on the applications increases I want to
monitor my resources so that I can anticipate when I will hit
bottlenecks on the db server.  That way we can do upgrades or
optimizations before our performance get's unacceptable.

We are monitoring cpu usage, memory usage, and network traffic.  I would
also like to monitor io utilization but am not quite sure how to do
that.  Does anyone here know to effetively monitor io to the raid (I'm
guessing that this could be measured with the raid management software
but was hoping their were some standard system commands.) or other io
that I should be measuring?

What bottlenecks could I hit and what are some good stats to check to
anticipate when I am moving towards one of those bottlenecks.

Thanks,

Rick