If you have a low bandwidth plan then I would suggest you to try installing MultiBit client and transfer your bitcoins from previous, unsynced wallet.dat to your new client and let it sync. It actually takes less time to sync because it is a thin client (it does not download the full blockchain).
#0 If someone has a problem with their car, dont suggest they take the train. Its rude.
#1 multibit uses a different wallet format, you cant just take the wallet.dat, shove it into the Multibit directory and hope that its working.
#2 Multibit HD does not allow to import private keys. What you suggest is not possible at all. Even if you export every private key. If you are refering to multibit classic, you should link to it.
Thin clients do not validate transactions, they just assume that the longest blockchain only has valid transactions. Moreover, thin clients do not help the network. Instead, they rely on full nodes (like Bitcoin-Qt) to do the heavy lifting.
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Now if you have a good bandwidth and want to contribute bitcoin by running a full 40GB node, then go ahead with Bitcoin-qt 
40 GB is not enough.
P.S There should have been a thread where the bitcoiners who want to use the newly downloaded core client, can seperately download the full blockchain in a compressed form (I think it can be compressed upto 20GB), unzip it, compile it with the client and then do the last syncing and most importantly, that file should be updated weekly. I think a legendary member should open a thread like that! Maybe theymos if you have time? 
There was a thread like that. Search for bootstrap.dat, but the blockchain can not be compressed to 20 GB, its roughly the same size when downloaded seperately. The problem however is that a seperate download can not be verified at the same time, thus it will take longer if you download the blockchain first and verify it later. Since 0.10. bitcoin core can download from several peers at once and should thus us the full bandwith available.
