Dreamcast 128-bit? No. Serious the SH4 isn't a powerful processor. It has 32-bit GPRs, with 128-bit registers in the FPU for 32-bit and 64-bit values.
Most of the things that people will discuss in this topic have already been said before, so I don't really feel like saying it again.
The xbox is not poweful enough to play commerical games of modern consoles.