Regardless, does it even matter? I doubt the feds will crack down on someone who downloaded a ROM of a 20 year old game for personal use. Unless you're commercializing it, there's nothing to fear. At least in my country.
Occasionally either the Feds or the entertainment industry will sue one individual the hell out of it to make an example. I'm just pointing out that when you said "it is legal from your own legitimate copy", you were wrong. It is that simple, from law perspective, backup and emulation are illegal.
there isn't a Xbox emulator that's actually on par with the PC ? (don't know for sure, since I don't have the first Xbox ...I'm just asking about it)
Emulation on N64? No.