Disclaimer : I'm very much a newbie here!
But I am on the path in my new job to figure this stuff out as well,
and went to PG Con here in Ottawa 2 weeks ago and attended quite a few
lectures on this topic. Have a look at :
http://wiki.postgresql.org/wiki/PgCon_2009
And in particular "Database Hardware Benchmarking" by Greg Smith
and
"Visualizing Postgres" by Michael Glaesmann
"Performance Whack-a-Mole" by Josh Berkus
--
“Mother Nature doesn’t do bailouts.”
- Glenn Prickett