We use Nagios, and we monitor the following server parameters:

• inodes and space available on each drive
• swap usage and load nominal, ssh available
• some custom checks to make sure the web application is up and running (pid and http checks)
• a check that PostgreSQL is alive

That's just an application server -- the other things you list are numbers that might be interesting, but probably won't signal that the server is going down shortly. Rather, they are stats that I'd probably look at once a week, but I wouldn't want to set any alarms for them.