This is a trilogy, Marc has already stated this. Killing Gwen in the last film is bad for (at least) two reasons:
1) ending the spidey franchise on such a sour note would be bad. This is spidey, yes he has some dark moments, but he is not a doom and gloom character. You want him to go out on a high note. Don't repeat the mistakes of SM3.
2) killing Gwen at the end of the last movie would be a disservice to the character. Part of what makes her death so important is the effect it has on Peter and how he copes with it. You just can't do any of that justice when it happens at the end (or even the middle) of the last film.
Here's how I would handle things. Introduce mj in the second film as the party-girl friend. She doesn't have to have a huge role, but she needs to be there. I'd personally make her and Peter at odds. Have her be slightly anti-spider-man, just to play off the idea that these two could never be in a relationship.
Kill Gwen off in the second movie. Yes, that means less time with Emma and the character, but that's the POINT. It should feel "too soon", because it's a death. It's supposed to be tragic.
Mj can have her turning point here, just like in the comics. Instead of being the immature party girl, we get to see that she actually has a heart and consoles Peter. This will establish their friendship for the third movie.
In the third film, they're still just casual friends, but Peter finds that he's starting to feel attraction. He hides it, however, because Mj still hates spidey (though by the end of the movie she will have come around), and Peter feels guilty for liking Gwen's friend (as well as her death - peters redemption would be the focus of this movie). But over the course of the movie, they become closer and help eachother through the grieving process. Throughout the movie, aunt may is trying to get Peter to go on a blind date, completely unaware Peter knows her. At the end of the movie, the last scene could be the revelation that the blind date isin fact MJ, with her famous line, then cut to black.
This way, we get Gwen's death, and even though she isn't in the third film, her presence felt and is still a huge factor. We also get MJ, but without a relationship lessening Gwen's impact. We also end the movie on a high note, showing that Peter WILL be alright, and that he's about to embark on one of comics longest and strongest relationships (damn you, Marvel!)