Is wallet.dat linked to the blockchain file?
From what I know, yes. The wallet and the blockchain data exist independently from one another. The wallet file keeps track of all transactions relevant to itself that appeared in the blockchain, as well as information about the latest block it has seen. If you use an old wallet on a newer blockchain, the client should scan any blocks that came after the waller's last synchronisation and update the information accordingly.

If you need Bitcoins urgently, you might want to use an eWallet until your client synchronises. Those are ready to run in seconds.