I have never been a gamer, but after having seen the graphics of the Wii, I kinda want one now. And part of me would love to find some vintage gaming system, like the Atari 2600 or the original Nintendo. So not knowing much about gaming consoles, I can't really say what accessory would be of much help beyond the stuff that a few people have already said: wireless control. I can't imagine that quoting what other people have said will help me win, but there it is.
cheers,
supervij