Elgato has an HD solution as well; not sure what its limits are, but I'm guessing if you have the bandwidth, it should be able to utilize it.
(Mac only, though)
http://www.elgato.com/elgato/na/mainmenu/products/EyeTV-HD/product1.en.html
(Mac only, though)
http://www.elgato.com/elgato/na/mainmenu/products/EyeTV-HD/product1.en.html