| If you're using a digital camcorder, I presume you're going to be using the firewire to transfer (in both directions?) the video between the camcorder and the laptop. If that's the case, you don't need an external capture device, as you can already get the video in digital form.
Obviously, I don't know the capabilities of your digital camcorder, so I could be making assumptions here that just aren't true.
Most capture devices won't do any rendering assist, being purely just capture/playback devices. This means that the processor in the machine will be used to do the rendering.
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