If you get rid of them you'll regret it later.
As the others said just put them in storage until you have room. If you have no storage room, make some.
Thanks for all the feedback so far guy's. The thing about it is I am not as a big gamer as you guy's. Even though i have a ps3 and a wii, my kids play with those two systems all the time. And at the time I was playing with the xbox just on weekends for about 1hr a day. I just play the nes, snes, genesis,tg 16 and mame roms on my xbox or maybe halo once in a while.
A while ago I was a hoarder on video games. I would just buy and not play. Now that I have 3 new systems and games I told my self just to keep one and get rid of two. That's why I dont want to storage anything.
I am a huge Genesis fan, but reading your description I think keeping the N64 would be best choice to keep.
You can easily play all of the Genesis and NES stuff on your Xbox. And since you say you've got kids I know they'd enjoy a lot of the N64 library, especially the first party stuff. Mario Kart, Diddy Kong racing, Super Mario 64, Banjo Kazooie, the Mario Party series, maybe even the old Tony Hawk games. Even if they've played the newest versions of say Mario Party and Mario Kart there is lots to be enjoyed.
Plus as others have said, the N64 doesn't emulate nearly as well. And overall the games aren't that expensive (maybe more than the NES and Genesis) but not by too much. Plus there are probably less N64 games that you'd be buying. Besides after selling the NES and Genesis you'll have more than enough money to buy any N64 titles you want.