The dreamcast uses hitachi processors, of considerable speed. I truly doubt that this, along with powervr video would be able to be emulated properly. on ANY x86 based processor in the next few years, let alone the 'measly' 733 in the xbox.
Search the web for dreamcast emus, perhaps someone started one for pc. Frankly, since sega is releasing software for other consoles, whatever game you want (sonic, phantasy star, SPACE HARRIER?!) will likely come out on the xbox.
add this to the SMS and Genesis emus that are out, and you're already set!