With that many updates to do, it’s best to run the system updates first, before updates for iWork, Garageband and other applications. Many people just run them all at once, and that’s good 99% of the time, but I like to try and minimize problems by running the most important updates by themselves.
Look for the large files that are 10.6 Updates; those are the best to run first and by themselves. Those updates will also have a tiny arrow next to the name that means you will need to restart while the update runs.
But before updates, what I like to do is do a restart of the iMac and run Disk Utility to check permissions. Start up Disk Utility (it’s in your Applications/Utilities folder) and select your hard drive in the left hand pane. That will take you to the First Aid tab, and there you will see a button that says “Verify Disk Permissions.” Click that and let it run. (It’s best to boot from the DVD to repair permissions, but this is almost as good).
After you have verified disk permissions, run Software Update and select the large OS X updates and run them. After your iMac has restarted, run the rest of the updates.
And be sure to turn on automatic updates in Software Update>>Preferences and set it to check for updates at least once a week. That’s the best way to keep updated, secure and not have so many updates to do at once.