Last year, Thor and Captain America: The First Avenger were released as the last films leading up to this. They both did fine, but they paled in comparison to Iron Man (both earned around $180 million each). Based on the anticipation alone, Avengers is definitely set for one of the biggest openings in history. The film has recieved critical acclaim (93% on Rotten Tomatoes, some calling it "the best superhero movie of all time"), and buzz from early screenings have been overwhelmingly positive and calling it a "must-see".
Harry Potter and the Deathly Hallows - Part II currently holds the opening weekend record with $169.2 million being the number to beat. So far, Avengers is one of four possible contenders to beat that number (along with The Dark Knight Rises, The Twilight Saga: Breaking Dawn - Part II and The Hobbit: An Unexpected Journey). But with kids in school on Friday (the film will likely target kids ages 11 and up), that might just prevent it from taking the record. Midnight grosses will come in Friday afternoon, and then we'll know for sure if it beats the record. Avengers also has made a tremendous boom overseas already, with $260 million in just one week in foreign markets.
Opening in over 10,000 screens at over 4,200 theaters, look for Avengers to clobber $162 million for its' opening weekend, the second-best ever. And based on how well it does in the next month or two, I can see a finish close to the $400 million mark.
As for the rest of the Top 5, it doesn't look that impressive, so I'll just toss out the rest in a list:
#2: Think Like A Man ($9 million, -47%)
#3: The Pirates: Band of Misfits ($7 million, -37%)
#4: The Hunger Games ($6 million, -45%)
#5: The Five-Year Engagement ($5.7 million, -45%)
#6: The Lucky One ($5.5 million, -49%)
#7: Safe ($4 million, -48%)
#8: The Raven ($3 million, -59%)
#9: Chimpanzee ($2.8 million, -47%)
#10: The Three Stooges ($2.7 million, -47%)