My laptop is an old IBM Thinkpad, 650MHz PIII, 128MB RAM. And it could handle capturing the video without any problems whatsoever. Obviously rendering transitions etc took longer than it would on a faster computer but thats to be expected.
And a built in firewire port would obviously be more conveniant, so there should be no reason to get a card. Just get a Firewire -> DV cable. If my laptop managed to capture the video at full res in real time w/o dropping frames, anything can.