the answers before me are great…. but remember that if you don’t already own the game, it is illegal to download. also, if you have an ipod touch or iphone and it’s jailbroken, you can play the games on it, if you SSH it over
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But GBA i can help you with! I often play pokemon on VisualBoyAdvance. You can see it here http://vba.ngemu.com/ or download here http://vba.ngemu.com/downloads.shtml You will need Direct X 7 or later!

As to the pokemon ROMs, they can be tricky to get hold of as Ninetendo often get the sites closed down for copyright infringement. The best place to get them at the moment is Dope Roms. Heres the link to Pokemon Emerald, but you can get the others on here too.
http://www.doperoms.com/roms/gameboy_advance_gba/Pokemon+Emerald+-+GBA.zip.html/124713/Pokemon%20Emerald%20-%20GBA.zip.html

I know you said sapphire and ruby but emerald is more or less the same but with a few more aspects (eg contests) and more content after beating the elite four.

If you find a decent emulator for N64 and DS you can download games for them from DopeRoms too.

Good luck!

PS I spoke to my partner and he says he tried the Project 64 emulator for N64 a while back and it worked great! Had a quick look and you can get it from DopeRoms as well http://www.doperoms.com/emulatorinfo.php?id=98

In terms of ease the GBA emulator and games are the easiest to start with. Simply install the emulator, and once you open it up click open and select the ROM file! Simple You can save by menus and shortcuts, and hold down space bar to accelerate the game to skip through annoying text
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PS according to law it is important that you own the original of these games before downloading the rom. Its up to you if you want to risk it, but felt i should warn you that you technically could be prosecuted if caught doing it