I'd think the likelihood of Avengers overtaking Titanic is slim, it'd have to make anoiher $100m during it's run in the theaters which I'm not sure it'll keep the momentum to do.
It all depends on how long The Avengers stays in theaters. If it sticks around for as long as The Dark Knight did (select theaters continued to run it a full three months after it was released on DVD), then it very well could make up the difference. Let's assume for the moment that the DVD/Blu-Ray comes to us in September. That'd leave it in some theaters until December.
It might stay in theatres for that long, but it will be on so few screens (and, mainly, discount screens) that it's pretty unlikely it will be able to make enough money to beat Titanic
. Still an impressive showing at the box office, though, no doubt about that. The Dark Knight Rises
will have a hard time beating it (especially with a running time close to three hours).