Some sound cards already have firewire ports in them, as well as some of the newer motherboards. My Audigy Gamer sound card has a very nice, gold-plated firewire port

So if you plan on getting a firewire adapter card for about $40, maybe add another $25-30 and get a kick ass sound card with the firewire port that's already built into it.
I'm using a 40GB 5400rpm Seagate hard drive, and have no problem, I capture video at broadcast quality running at 3.62 MB per second (which is standard DV size as well).
The biggest thing you have to worry about is capture, because you don't want your computer to be too slow or it will drop frames during capture (the hard drive mentioned above is plenty, and works flawlessly, just yesterday I captured over an hour of continues video with not even a single frame being lost). For editing you don't need a fast machine, but slower machine will take longer to edit scenes, but it will work just as well.