if you can back up the whole iPhoto directory (in your home/Pictures directory) in one place, this would be the safest bet. When i need to reinstall my system, i usually just back up that directory....and everything is fine
alternatively you can manually select pictures and drag them to a folder on your desktop, backup that folder to cd, and then delete those photos from iphoto. Then add them again anytime you want to see them.
hope this helps.