Doom 3 will be VERY scaleable (sic?), in that if you turn everything off and run at 640x480x16 on that system, it will RUN. Meaning, you could get through the game without it bunking out complaining about some unknown API call.
I wouldn't expect framerates to get any higher than 10 at any point though, and anyone with this kind of a system who buys Doom 3 is totally out of his mind. You could practically double the power of your computer system for the same price as the game itself.
Obviously, the more power you throw at this engine, the better it looks, and the more fps you get. From screenshots and that intro video, I doubt the machine exists today that would push the Doom 3 engine to its limit, but this is the way Carmack designed it. He planned on this engine to be around for about five years, and in five years we'll probably be playing on 7-10Ghz processors, not even taking into account all the improvements we'd see on graphics, APIs and processor design.
I think Doom 3 is going to look awesome, but for gameplay we'll have to play Half Life 2, Deus Ex : IW, and wait for some other developers to start building something with the Doom 3 engine, since gameplay hasn't been a staple of Id since the original Doom.