Is there a way to back-up your saved games on the PS3 in case they are overwritten?
To back up the save from a particular game there is an easier way than backing up your entire hard drive. Just use a thumb drive in the USB slot, and go to the "Saved Data Utility (PS3)" item in the PS3 menu (it's above "Trophy Collection"). Then select a save file and press triangle, then select copy. You should see the thumb drive listed as a destination.
Also, saved game backup is one of the features included w/ subscribing to PlayStation Plus
Yes, you can back up your PS3 data to an external hard drive. Watch the following video to show you how to do it from the system menu.
Be aware that backing up saved games to a USB thumb drive, and potentially to an external hard drive, doesn't always guarantee saving all of the associated data-- games with online components, like Rock Band, can restrict the ability of a user to backup the data associated with the saved content. I found out about Rock Band's issue the hard way...
If you use the full PS3 backup it appears to back up protected game's saves also as I have Dragon Age Origins and after upgrading my PS3 hard drive this weekend I still have the game data. It's possible that having the Dragon Age Complete collection that was released is unprotected making my anecdotal evidence invalid.