Public Key + Private Key enough to backup? I invested in bitcoins and unfortunately I have problems to understand the system. Perhaps you can answer me the following questions?

1.a) I know my public and my private key. Do I still have to backup my wallet.dat?

1b) If I need the wallet.dat: There is no point in saving my public and private key, if I need the wallet.dat anyways, right?


If I secure my wallet.dat with a strong password, can I put the wallet.dat just on dropbox and send it to a few friends or do I have to keep the file more private?
Is the decripting function of Bitcoin-QT enough?
Am I okay with these actionsteps:
Encrypting my wallet with Bitcoin-QT and writing my password on paper
Writing my public key and private key on paper
Sending the bitcoins from mtgox to my public address
Using the backup option of Bitcoin-QT and send the backup to dropbox + a friend I trust


Please let me know if I need more steps or I can take a step out.

Thanks!
Knowing your private and public keys is enough to be a backup of that address. If you only have one address all you would have to do is create a new wallet and import that private key.

Why backup the entire wallet.dat then?


Because restoring the wallet.dat saves you the step of importing the keys from the command line. Most people find it easier to backup a file than understand the details of keys and addresses.
Because many users have a lot of addresses. If you have a hundred addresses, restoring one wallet file is quicker than restoring each private key.
Because of change addresses. The default client ends up creating a lot of change addresses automatically. You may think you know your private/public key pair, but if you've conducted some transactions in the standard client it is likely that you also have a bunch of change addresses that you aren't directly aware of and aren't displayed in the GUI.
Because of the address buffer. The standard client keeps a buffer of 100 addresses that it intends to use. This means that even restoring a wallet.dat can sometimes save you even if you don't have an up-to-date backup of your wallet. (Because the transactions/addresses that aren't in your wallet are probably in that buffer of addresses.)


In short, yes, the private/public keypairs are the important thing and if you know them (say by keeping them in a paper wallet or encrypted somewhere) you can recreate your wallet. But there are lots of mistakes you can make (such as forgetting change addresses) if you are manually tracking keys. If you aren't certain what you are doing, you may find backing up the wallet.dat as the easiest approach.