You'll want an internal IDE most likely (assuming that you've got a pre-made computer like Dell, HP etc.). The larger the hard drive the better, and get one that's at least 7200 rpm, preferably a name brand (go cheap and you might regret it when it dies 10 minutes after the warranty expires). You can pick up a 200 gb drive for well under a dollar a gb nowadays!
BTW, if your other drive is small (say, 40gb), it'll fill up really fast if you're putting movies on it. As an alternative, you can buy an external hard drive that's USB plug and play (just plug it into your machine and go!). Those tend to be a bit more expensive than internal drives.
If it were me, I'd go with the internal drive. IDE
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