If we're talking strictly legal, then it depends on the country obviously, but mostly, the laws pertain to HOW you aqcuire the copy. The legal issue with emulation is commonly not the actual act of emulating, but rather that many people download the game illegally to do so. It is in fact illegal to download a copy of something, even if you own a copy of it yourself (at least most places). So even if you owned a CD with your favourite music, you can't legally download those tracks from The Pirate Bay or whatever, because the act of downloading it is illegal in itself. Same goes for emulated games. However if you are able to rip the file from the disc and onto your HDD/SSD by yourself, instead of downloading it, you are free to do whatever you want.
Of course, that's just about what is legal. When it comes to ethics, I don't *really* see a problem with pirating something you already own, but if you want to be completely legal about it, you have to rip the game from the disc/cartridge itself.
