If you notice, the minimum requirements are a GF1. Heck, it wasn't THAT long ago I remember him saying that a GF3 would run it nicely. The GF3 isn't that bad of a card at this point. My brother still uses my old GF2 to play the current games at 800x600, and he says it runs things fine.
You have to remember, that an XBox only has to run a game at 640x480. At that low a resolution, the 700MHz (or whatever) CPU in the xbox should be able to run it with some of the features turned off. Unlike a PC, the XBox ONLY has to worry about 1 thing: the game. It doesn't need to worry about background processes and other apps. I'm a PC user, but saying that the XBox could not handle DOOM3 is a little harsh at his point.
Honestly, nobody KNOWS how DOOM3 will run yet, let alone on an XBox. The Alpha shouldn't be a basis to prove minimum requirements, as it hadn't been optimized yet. The MOST you can say about the Alpha is that IF your machine did a good job running it, then it should run the final fine. But if it ran like crap, it still might be able to run the final version at an ok frame rate.
Let's play the wait-and-see game.
"Space. It seems to go on and on forever. But then you get to the end and a gorilla starts throwing barrels at you."
-Fry, Futurama