Q: Is it possible to update my operating system from 10.6.6 to 10.6.8

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Of course. Open  > Software Update, and you will be prompted to install 10.6.8. Another way would be to download it from the Apple website > http://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US

Just to add a little to mende1's post; you may need to add software updates after applying the Combo updater.    I think there are two or three security and iTunes updates not covered by the Combo.    If you do follow the Combo route just go to Systems prefs > software update and they should all show up.