In the original client, when you make a transaction, it takes money from "wherever there is funds."  I don't know how the decision is made as for what address the funds are taken from, but:

Let's say you received have 10 BTC on one address, say address "1addressIrecieved10BTCon"
then you send 2 BTC to "1addressIamSending2BTCto".
Therefore,
10BTC are taken out of "1addressIrecieved10BTCon",
2BCT are sent to "1addressIamSending2BTCto",
8BTC are sent to a "change" address, say "1addressInMyWalletButHiddenFromMyView"

This means that the address you took funds from, i.e. "1addressIrecieved10BTCon", is now empty, while the 8 BTC that are still available for you to spend are in a change address that actually belong to your wallet, but that you will not see directly in the bitcoin client.


If your funds are spreader over multiple addresses, I DO NOT KNOW how it determines what address it will take funds from.  My GUESS would be that it merges the smallest amount until it reaches the amount you want to spend, though it would make sense for it to tend to avoid spending funds from transaction that received fewer confirmations.

One sure thing is that the only control you have over where funds are taken from and where they are going is limited to the one address you wish to send funds to, and the amount that will be send to that address.  Where the funds come from and where the change goes is out of your hands, unless you know how to use the command lines to "micro-manage" your transaction, or have a client that will allow you to do it.

If you are comfortable enough, you can go in "Help -< Debug" (menu) and then in the "Console" tab and type "help".  You will see the commands available.  You can then use the "sendmany" command in order to select the address you want to send funds to and the "account" you want to take the funds from.  The "account" (which is neither a wallet, nor an address) should match the label, but change (e.g. the 8 BTC above) addresses could have been created in the same "account".  The funds would then be spitted over many addresses, but under the same "account" (or label), so the "coins" would still have come from that same first address, although the funds will have moved through intermediate address in the process.

An account is a subset of addresses withing a wallet.  An account may contain only one address, or many.

Another workaround would be to create separate wallets and send the amounts you want to each wallet, then decide which wallet you want to spend from, but I almost get upsets just to think about that idea.

I hope that helps.