Price no object? Very high quality? Canopus ADVC 300? (Or a 110 if that's a little TOO expensive)
Price somewhat of an object? Pinnacle MovieBoard or MovieBox?
Or, if you have a DV camcorder that does analog passthru, that's a good route to go as well.
But just realize: There aren't many capture devices out there that are made to work with the XBox, so whatever you decide to buy, be sure that it not only works well with Vista, but will capture potentially "weird" sources such as you can get from video games.