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It won't drain any faster because the power requirements are about the same as any other HDD. I would go with the Samsung which is a bit faster and surely will fit in any computer if you ever decide to move the drive to another computer.
 
If I use time machine to backup my current HDD to my Time Capsule, when I upgrade my HDD what do I half to do?

I think I will have to install the Leopard DVD and use the Migrate feature, as if it was a new computer? is this correct?
 
I would use SuperDuper or something like to clone the HDD to the new one and then install it. Of course you would load up the Restore DVD, select Restore from Time Machine, connect to the network and restore from the Time Capsule, but this method might be slow. No need for migration assistant.
 
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