I should say the only flaw with this debate is we don't really know if the wii is "emulating" said system or if said games are being reworked to play on the wii... If they completely rework the game(not really all that hard actually especially on 16/8 and lower bit games.) then the only limitation with Dreamcast would be the fact that Sega used GD-Roms(1.2GB)
As far as emulating, Wii could but it'd be pretty sketchy with the wii running off a broadway based chip estimated at 729Mhz and Dreamcast having used a 200Mhz chip. The thing is the processor speed isn't the end all of speed. Architecture affects processors greatly. Do either the wii or Sega Dreamcast run with power saving measures? Generally power saving cuts back on processor speed so while advertised speed may be 2.0Ghz on your laptop it may be squeezing out 80% of that.
