What is a better option, to extract the private keys and just save them and import them to a future client when needed or to save the entire wallet.dat?
If a wallet has multiple imported private keys, how many private keys will be exported.
IfI import the private keys to the new client/wallet will the old created addresses show on the new client once it has sync?
Different client have different backup needs and approached.
With Bitcoin-Qt, the wallet.dat contains every address plus a key pool for the next 100 unused addresses. So you can back that up and at some point in the future restore that file if needed.
Other clients might use a deterministic wallet. So with those only the master, spending of all BItcoin addresses created from that master is enabled.
A single address can exist in multiple wallets, that's not a problem. What is a problem is if you try to spend from using a Bitcoin address that exists in multiple wallets simultaneously. That's when you can gunk up your Bitcoin-Qt wallet.
But importing a private key from an old wallet.dat, and then later having recover that old wallet.dat is not a problem. The funds will appear as being spendable in both. Just remember to only have one running at a time. (And don't spend until the client has sync).
