Basically that's right, as long as your server is supporting the "If-Modified-Since" header. A 200 response is the server just saying "ok, here's your document." A 304 means the requestor sent an "If-Modified-Since" request, and the server reponse was "it hasn't changed," so the document wasn't actually sent.
If your server isn't set up to support that, you'll only see 200s (or some other response for other situations), and never see a 304.
A related thread: [webmasterworld.com...]