Does my Bitcoin-Qt wallet backup file have "everything" I need (private key)?
I mean, is that the file I would copy and store offsite in case my house burns down? Does it have my private key encoded in there?
You only need to copy the
wallet.dat to backup your private-keys. Do this while Bitcoin-Qt is not running. All other information will be deduced from the blockchain once you re-import the
wallet.dat except any custom labels you have given to addresses.
Of course, if you encrypted the
wallet.dat you will also have to remember the passphrase.
Your bitcoin.dat file contains your private key(s) as well, however I would suggest you extract them now for the addresses that hold money (if any), if empty then it's not much of a use, unless you accept donations there or have a business set up to that address.
How to extract your private keys for addresses that hold at least some money: