I don't think the N64 is powerful enough to very accurately emulate a Gameboy or a NES, though it can do so imperfectly. The N64's strength lies in it's 3D hardware rather than it's CPU (I believe, though I'm not technically acquainted with the details), and for emulation of 8 bit systems it's the host hardware's CPU that matters, the 3D hardware is not used at all.
If you get a used original XBox or a Wii, then you can softmod them to use them to play emulators. I've not tried a Wii, though I've heard it's great for emulators, but I have a modded XBox, and it's *fantastic* for emulators. Emulation of many 8 and 16 bit systems is as close to perfect as makes no difference, and includes standard emulation features such as save/load any time by the use of snapshots, playing all regions of roms and hacks, and re-definable controls. Plus many emulators have bonus features such as cheat support, screenshot or even video support to show you what the game you've currently highlighted in the emulator's Load menu looks like, and even fast forward or rewind. Fast forward lets you skip through boring bits, such as lengthy cutscenes, or boring travelling sections of a game, and rewind allows you to rewind the game to an earlier point (say ten or twenty seconds) in case you make a mistake and want to retry (for as often as you like).
Emulation of anything later than 16 bit on the original XBox is hit and miss. Some Playstation and N64 games play fine, some don't play at all (or are so bad that they may as well not play on the emulator), and many have minor or major glitches. The same with Atari Jaguar games, and other emulators of 32 or 64 bit systems. But Gameboy/Gameboy Colour, and NES games (and SNES, and Megadrive, and Atari ST, and ZX Spectrum, and C64, and MAME (arcade), etc) play fantastically on a modded original XBox. Plus if you replace the XBox's standard hard drive with another one (I have a 500GB one in mine) then you can play everything directly from the hard drive, including your XBox games).
Note that I mean the original XBox, of course, and not the XBox 360 or the XBox One. I don't know if any emulators are available for those (because the original XBox does everything I want, emulation-wise).
