Upgrading from 10.5.6 to 10.5.7 from the apple menu does the upgrade to Safari automatically. You may have upset something by running a Safari 3.2.3 upgrade over the top. In your scene i would download the 10.5.7 combo updater
here, and run that over your current set up and that should sort Safari 3 and then try your luck with the Safari 4 beta.