interesting question but I think you have to narrow it down some, because of the different ways of emulation;
mame-type: complete disconnect from hardware, can run anything on anything but costly in resources. (I remember running a 4MHz z80 game on A1200 28MHz at around 1-2 fps)
wine/shapeshifter type: can run at full speed (probably need MMU) but must be same cpu.
mix (psx emulators etc.): full cpu emulation while passing things like 3d functions to a 3d card.
