when I send money from my wallet, which address shows up as "sender"?
Bitcoin doesn't really have sender addresses.

Every transaction consumes some of your coins, and creates one or more new coins, potentially assigned to a different address. What the receiver can see is the transaction, and he can trace which coins it spent, and which addresses those coins were previously assigned to. Some programs/sites use the first such address as sender, but this is not recommended. It doesn't say who sent the money - only who previously controlled it (which may for example be a shared webwallet).

The reference client always considers its entire pool of (confirmed) coins as source for creating transactions. The recently (0.7.0) introduced raw transaction API allows fine-grained control over which coins are used as input, but does not have a GUI (yet).