Seems to me the volume of bitcoins held in mtgox.
If you go to bitcoin.clarkmoody.com and set "group by price" 50btc and increase max rows you will see that there are about 131000 of bitcoins in the order book waiting to be bought for fiat.

It is not clear for me what you are trying to achieve. Do you want to trade BTC/LTC, or monitor BTC/USD and LTC/USD ratios?

1DdsWtwaNP5iSfKXzwqFYVF4EBoEuiptft - if that code is given to you by mtgox it is not your address. You can't control it from your computer. It is just a token for receiving  payments to mtgox made by you. You can technically pay from mtgox to someone, but the money will come from different address and I'm not sure if sending btc to a third party is allowed in their ToS.

Don't try to synchronize your wallets across computers unless you absolutely know what you are doing.
Original bitcoin-qt client sends leftover coins to a new address every time. Which is counterintuitive and you can end up loosing your coins.

You can setup a shared public address across your computers and after each transaction send any remaining coins (your "cange") back to your public address after transaction (you don't need to wait for 6 confirmations to do so). Or I believe there should be a client which does this automatically in a same transaction. That way you will have the same amount of money on all machines.
To do that start with a fresh new wallet.dat file and request a new address for new account.
Close client. Copy wallet.dat to other computers. After receiving  coins to that address all your computers should see the same amount of btc.
 

Online wallet is the most convenient but least safe method of storing coins. Companies behind them are not as reputable as the banks, and since btc are commonly "virtual commodity for geeks" company have little legal responsibility for "loosing" them.
I personally would not hold there more than 1btc for a quick trade.

As the largest and well known exchange they should know their customers. Otherwise their bank account will closed for AML investigation.
I wonder why it still did not happen.
If you need privacy try OTC trade with cash.

I made a wire transfer to mtgox. Can't say it went completely smooth.
It took 3 weeks and price rose from during that period $12 to $30.
But I believe it is the safest and cheapest method.
The drawback is that it is slow and requires pretty large sum for the starters ($1000+).
I think Bitinstant.com is an easy and safe option for small and fast exchange.

I've been lurking on forum for months yet when I needed to PM I realized that having an account is not enough.
Too bad I was not lurking on a newbie board...