1) "Does she like (card) games?" Is probably the most important question to ask yourself before figuring out the best way to introduce the game. If "no" stick with Base and don't think any more about it.
2) Is there another couple (a friend of yours + wife, or a friend + sig other of hers?) you can introduce to Dominion at the same time as well? Might be more of a social event. In fact, host a dinner + game party. (Eat dinner first, then play the game.)
I have found that it's usually better if more than one person is unfamiliar with a new game when trying to introduce it. There is less of a feeling of "oh, I'm slowing the game down because I'm new and don't know what any of the cards say."
3) When actually introducing Dominion (or any other game), keep it simple, rules focused, and restrain yourself from trying to point out any (even obvious) strategies until the rules are learned, or you get asked. From the time you start explaining to the time you start playing, it should be less than 10 minutes.
Keep the pitch along the lines of "The goal of Dominion is to get the most points possible. [Explain/show Victory cards.] You do this by buying cards [point to the Treasure and Action cards, but do NOT explain them each in detail] and building a deck. The person who's deck has the most Victory points at the end of the game wins. You do this by 1) playing an action, 2) buying a card, and 3) discarding all your cards and drawing a new hand [show a simple example turn, emphasizing you will be using the newly bought cards "right away." Show a reshuffle.]. This is more of a deck building card game rather than a card game that is played using a deck of cards." (5-7 minutes at MOST).
Then once the rules (or the important ones) are explained move onto SIMPLE strategies so she knows where to begin: "You could buy an Estate on the first turn, it's worth one point, but then you would never be able to get those cards worth 6 points. So you need to buy other cards to improve your deck (even thought they are not worth any points) and give yourself the ability to buy bigger and better Victory cards." (just 2-3 minutes) Then emphasize the game ending conditions, set up the first game, and begin!
After the first game, emphasize common problems you see crop up (not buying enough money, not having enough +actions, etc.)
Good luck.