If its an OTA update, your data SHOULD be fine. However, its always a good idea to make a backup just in case. Your texts can be backed up using a number of apps, SMS Backup and Restore is one I use.
Your contacts and calendar SHOULD already be backed up to google, provided your contacts are Google Contacts and not Phone contacts.
Apps you purchased will be remembered, as long as you sign into the same google account. Apps that were free and downloaded wont be remembered, so it's best to make a list of the ones you already use, then you can just re-download them. There is no way to backup app's data unless you are rooted. So for example, if you play Angry birds, you will lose your saved games.
But again, this is all only if the update does a full wipe, which updates usually dont.