[quote name='Genocidal']If you're confident in your abilities with computers you can make your own for about $50.
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Or, if you're not confident in your abilities with computers, you can go on ebay and find 360 hard drive upgrade kits which provides you the 120 gig hard drive, pre-flashed so it's ready to work, as well as whatever special screwdrivers you need to open the old hard drive. With the hard drive pre-flashed for you, it looks like a piece of cake to make the switch yourself.
Of course, this is assuming you have an old 20 gb hard drive lying around, since you do need the old shell to plug the bigger hard drive into.