It will "theoretically" be at least 60% more powerful than the Xbox360. However it will also be more expensive at launch than the Xbox360.
And don't foget, that extra power is theoretical and probably won't make much difference to the level of gameplay and/or graphics in games.
I think it will be interesting to see whether or not games developers continue to produce games for the Xbox360 come the launch of the PlayStation 3, as it is very similar to the Saturn in that the coders need to be very comfortable with probramming for multiple independent CPU's (one of the major downfalls of the Sega Saturn and certainly one of the things that killed it).
Mind you, if Sony's "Emotion Engine" is anything to go by, the PlayStation 3 will be utter crap anyway, good on paper and built up to be awsome, but lacklustre when it comes to real power.