Monitoring postgres slowdowns

Hello,
is there any way to "look under the hood" when slowdowns occur? We
have a tomcat / postgres site with each app having it's own server.
The db machine is a dual CPU / RAID 5 / 2GB RAM box running RedHat
Linux 7.1 and Postgres 7.1.3
The two machines are connected via a hub on which no other machines
are present (i.e. private link). shmall is set to 805306368 and shmmax
is 536870912

We seem to have daily slowdowns, and the only tools I know of are top
and ps, which are pretty general and only tell you when something is
cranking along. I'd like to better be able to 1) determine if indeed
something strange is happening with our postgres install and 2) what
it might be. I could find no pointers in the faq.

Out user load isn't very heavy (max of 200 users), yet occasionally
things just crawl. Looking at the tomcat machine shows most memory
free low CPU usage, so all signs point to the DB machine - but how to
tell if something's wrong / what exactly it is doing at the moment?
It's getting frustrating because when it happens everyone looks at me,
and I have no idea how to pinpoint what's happening.

(Also, we are doing a nightly vacuum --analyze (we tried doing hourly
vacuums on 6 of our update-heavy tables, but that slowed things down
too much))

thanks,
-Steve