I ran out of space while synchronizing, how do I get my private keys & find my balance? OK, my most immediate concern is that I made three ~$500 bitcoin purchases this weekend and downloaded the Bitcoin-Qt client, but ran out of HDD space just before it finished synchronizing the BlockChain, and now I don't even know what my BTC balance is, if my value is lost, or what.

Now I don't know what to do.

I was at least bright enough to have made a screen print of the three bitcoin addresses from the client and also of my BitInstant orders (while they show the destination address and $ amounts paid/recieved, they don't list the amount of bitcoin I assume and hope that I received), but I can't find my private keys in the client, though it appears that I am able to Send Coins from the tab of that name.

I don't even know what my total amount of bitcoin is since Bitcoin-Qt hasn't finished synch-ing up, and I'm just sort of stuck without either bitcoin or fiat since my HDD is down to only ~200MB left with 1,746 blocks remaining.

What should I do?

I read that the Multibit and Electrum wallets seem to need less computer resources, so I was thinking/hoping that I could maybe transfer my bitcoin to one of those wallets, or maybe to the StrongCoin online wallet or other secure recommendation as an interim step, but I don't even know the precise quantity of bitcoin that I should have in my bitcoin addresses, or if I can even send them without knowing my private keys.
If you know what bitcoin address your bitcoins are coming to (and you should know if you expect to get your bitcoins), you can lookup balance on blockchain.info like this:

http://blockchain.info/address/your-address

replace your-address with your actual bitcoin address, for example here some random address i found: 

http://blockchain.info/address/1LNx3azmdKyH43UsJ6DK9oUrTbkPUr6UJg