Let's say I have a WaveRace64 cartridge sitting in my desk drawer (which is actually true, my favourite N64 game!) - am I breaking any laws by downloading an image of that cartridge for use in a N64 emulator? I'd be surprised if I am.

I'm allowed to rip my own CDs to MP3... if I were to download the ripped MP3 of a CD that I already own, what difference is there (other than the fact I haven't had to bother doing the rip in the first place)?