In particular, a number of minor security issues were fixed, so even if you don't plan to make any major upgrades in the near future, we highly recommend upgrading to the latest stable package for your version.
You can find all detailed information about the releases in the release notes for Moodle 1.7.2 and Moodle 1.6.5, and of course, all the downloads are available on the Moodle download page.