This thread immediately caught my eye because I was in your wife's shoes about two to three years ago. My husband, a great guy, was a 2ed player. Before we got married, he once tried to introduce me to the concept, but I found it a little weird. We were sitting around one day and I expressed curiosity about the game because it was something he obviously liked a great deal. We decided we'd roll up a character for me, even though there was not a game going on at that time. While I enjoyed tossing the dice (and still do!), I found the character creation process to be a bit tedious. We tried an hour of roleplay, just the two of us, but I found it a bit ridiculous and came away with the sense that D&D was not for me. I think I felt uncomforable because it was just the two of us and I had no clue what the heck I was doing.
Fast-forward to 3.0's appearance. We bought the books and founded a SMALL group of very good friends to play with. I was immediately hooked. The key, for me, was playing with a small group with whom I felt comfortable. D&D is ultimately a social game and had we started out playing with a larger group or with a group of only my husband's friends, I would have felt alienated and probably would not have enjoyed the game as much. I recommend a small group of mutual friends. It helps if someone else in the group is a newbie, that way your wife won't feel as alone. Everyone else here has recommended neat adventure ideas, but I really feel the best way to get off on the right foot is to consider the social aspect of playing. To this day, I still love hosting a game. I like having friends over and I like doing some cooking or baking before they show up.
As for which sort of PC to play, my opinion is to begin playing a core class and a non-spellcaster. By playing a core class and a non-caster, it may be easier for your wife to understand the rules system, which, in my opinion, will greatly enhance her playing experience. It's always nice to know what the heck the other long-time players are talking about, rules-wise. If your wife insists on a spellcaster, encourage her to take Bard or Sorcerer. I started out playing a Cleric, and it really took me a looooong time to master that huge spell list. Not to mention that it would take me forever at the table to select which spells I was going to memorize for the day. I think the best character to play would be Ranger...they are great at fighting, useful in other non-combat situations, and grow into spellcasting. This, of course, only works if you are going to be starting out at first level.
Starting out at first level is also the best way to go; she'll gain an appreciation for her class and will be able to grow into it as time passes. It's also easier on becoming familiar with the rules.
We started out by playing The Sunless Citadel, and it was enjoyable to all, including the long-time DM. I thoroughly recommend it for your wife's first adventure. I wouldn't necessarily continue on past Sunless Citadel because I don't consider the other adventures in that path to be the same caliber as the first.
Last, maybe you could show these posts to your wife and get her input? She might tell you which opinions match her own, thus giving you a better sense for how to make the game fun for her. D&D is one of my favorite hobbies. Best of luck to you two, and hope you accrue many fond memories playing together.
edit: one more thing...get her her own PHB. very important, that.