Robert Downey Jr. in 'Iron Man 2'There's no question that 'Iron Man 2' will be the unstoppable box office victor of the first weekend of the summer. The only question is: how big will its victory be? Will it be big enough to break the opening-weekend record of $158.2 million set two years ago by 'The Dark Knight'?

In 'Iron Man's' favor: It's the sequel to a huge critical and commercial hit. It's the first big movie of the summer. It faces no real challenges from other new movies. It's opening on a record-busting 4,380 screens. One hundred eighty two of them are IMAX screens, so 'Iron Man' can count on some hefty surcharges. It has Robert Downey Jr., whose bankability has greatly increased since the first 'Iron Man' two years ago. It has a $100 million marketing campaign. It has appeal to all demographic groups, not just young fanboys. It has the continued comeback of Mickey Rourke, as a buff villain. Oh, and it has Scarlett Johansson in a catsuit. Robert Downey Jr. in 'Iron Man 2'There's no question that 'Iron Man 2' will be the unstoppable box office victor of the first weekend of the summer. The only question is: How big will its victory be? Will it be big enough to break the opening-weekend record of $158.2 million set two years ago by 'The Dark Knight'?

In 'Iron Man's' favor: It's the sequel to a huge critical and commercial hit. It's the first big movie of the summer. It faces no real challenges from other new movies. It's opening on a record-busting 4,380 screens. One hundred eighty two of them are IMAX screens, so 'Iron Man' can count on some hefty surcharges. It has Robert Downey Jr., whose bankability has greatly increased since the first 'Iron Man' two years ago. It has a $100 million marketing campaign. It has appeal to all demographic groups, not just young fanboys. It has the continued comeback of Mickey Rourke, as a buff villain. Oh, and it has Scarlett Johansson in a catsuit.

About the only thing working against it is its advance buzz, which is not so hot. Middling word of mouth has been followed by lukewarm reviews. A letdown may be inevitable after the high expectations bred by the first movie. That one opened with $102.1 million on the same early-May weekend two years ago. This one will surely surpass it, but even studio insiders are predicting a maximum take of $135, according to Variety. That seems overly conservative to me. My prediction: $145 million.

Bayarjargal in 'Babies'The weekend's other major release, the documentary 'Babies,' following a year in the lives of four tots from around the world, has been getting very strong advance buzz. Besides being a smart alternative to 'Iron Man 2' for the Mother's Day crowd, it has irresistible cuteness going for it as well. Opening on a modest 534 theaters, it should earn about $3 million -- not enough to break into the top 5, but strong for a documentary.

As for the rest of the top 5: Last week's No. 1 film, 'A Nightmare on Elm Street,' isn't expected to have long legs. After it opened with a weaker-than-expected $32.2 million last week, poor word of mouth should lead to a two-thirds drop-off this weekend. 'How to Train Your Dragon,' still going strong in its seventh week, should earn enough to top $200 million this weekend. Expect modest drops for this weekend's romantic comedy options, 'Date Night' and 'The Back-Up Plan.'

Top 5 predictions:

1. 'Iron Man 2,' $145 million
2. 'A Nightmare on Elm Street,' $11.5 million
3. How to Train Your Dragon, '$7 million
4. 'Date Night,' $5 million
5. 'The Back-Up Plan,' $4.5 million

• Follow Gary Susman on Twitter @garysusman.