Xbox is a LOT easier to code on because it's basically Windows. The GC/Wii make you have to boot into your own code, which is much harder to do efficiently without being an official developer. That said, with the power of the Wii (though it's small) there should be no problem emulating the n64 flawlessly. Most people say you need 7x the processing power to emulate. That's just a saying, it's not necessarily accurate, but it's a good way to see what's possible: The N64 clocks at 93.75MHz, the Wii at 729MHz, so it's definitely powerful enough.
However, hackers haven't yet figured out how to create Wii homebrew (that is, homebrew that will boot in Wii mode instead of GC mode). That said, until they figure it out there won't be an N64 emu.