How I save my text messages iPhone

No, you can't save messages to iCloud.  Messages are included in your backup when you sync, but cannot be read separately (they are only there to restore your phone, should you need to).  If you want to archive your messages you can either take screenshoots of your conversations and email them to yourself or use 3rd party software such as PhoneView (mac) or Touch Copy (pc or mac), which will save your entire message conversations.

Does that mean if for some reason I had to replace my iPhone.. I can restore my texts back onto my new phone?

No, you can't save messages to iCloud.  Messages are included in your backup when you sync, but cannot be read separately (they are only there to restore your phone, should you need to).  If you want to archive your messages you can either take screenshoots of your conversations and email them to yourself or use 3rd party software such as PhoneView (mac) or Touch Copy (pc or mac), which will save your entire message conversations.

Yes, messages are stored in the backup.  Use this guide to transfer your data and settings from your current phone to a replacement phone:  http://support.apple.com/kb/ht2109.  You will still have to sync with iTunes to transfer iTunes media (music, etc.) to the replacement phone as it is not included in the backup.  To see what is included in the backup see http://support.apple.com/kb/HT4946).

Does that mean if for some reason I had to replace my iPhone.. I can restore my texts back onto my new phone?