Xbox 2 is 4 years ahead of Xbox 1 on the technology curve, which means it's roughly 4 to 8 times more powerful. (Moore's Law is 2x every 18 months) in each respect.
If you look at the rumored specs you'll see that:
CPU: 3 times as many CPUs
CPU freq: 5x faster (or whatever 3.5 / .733 is)
Memory: 4x more
GPU: Way faster
Hard disk: faster, larger, but maybe optional.
DVD: same capacity (Or bigger if they go with HD-DVD or Blu-Ray.)
Now if you're a marketer you multiply all those scale factors together and no matter what result you get you say it's a thousand times more powerful.
If you're a developer, you just take the average 4x improvement, and then derate by 30%, so you say it's about 3 times more powerful.
If you're a developer who needs a hard disk, you say the Xbox 1 is more powerful.