as everyone and his dog knows, Sony has been saying that PS3 will be a 1000x more powerful than PS2. they've said that since 1999, before the PS2 came out. Sony also said PS2 is 200~300x as powerful as PS1. I think 215x was an actual figure. obviously Sony is not going by one specific spec (i.e. FP performance) but overall rendering performance.
I have never seen Microsoft say Xbox2 will be 1000x more powerful than Xbox. or any other figure.
I think a reasonable increase would be 10~20x Xbox's graphics performance, for Xbox2. In comparing NV2A to R300, we have a 2-3x leap. if R420 is 2x faster than R300, and R500 is 2x faster than R420, and if R500X (for Xbox2) is 2x faster than the normal R500 for PC, plus some extra efficiency, we might get a 10~20x performance leap from Xbox to Xbox2. a 50x leap is probably unlikely. bandwidth will go from 6.4 GB/sec to probably 50~60 GB/sec.
now, putting speculations aside, all I want out of Xbox 2 is to have IN-GAME graphics that are just as good and just as smooth as the pre-rendered CGI that MS showed for Xbox1's announcement at GDC 2000. (the Robot demo, and the Afro Thunder demo)