I used to run a create-your-own-quiz django powered site with 6k uniques/day(in the last few days), with no cache, and it barely touched my 360 vps. You can see some nice graphs here: http://slig.imgur.com/linode
From my experience looking at those graphs, I see that the iorate is pretty low and that the cpu is almost idle. Checking "top", I see that the RAM was sufficient, as there were almost no swap use.
I also use chartbeat.com to see how the users are interacting with the site in realtime. They provide also two useful metrics: user load time and server load time. Looking at those, I can see that the user experience is pretty good, and that the server is up and responding very quickly.