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.