By Nicole Sperling
Updated July 30, 2009 at 06:06 PM EDT

Judd Apatow has become a household name thanks to his past two outings: Knocked Up and The 40-Year Old Virgin. But where both comedies went on to become $100 million+ box office successes, his latest endeavor, Funny People, is much more of a wild card. The R-rated flick still features some of Apatow’s unique brand of comedy, but hidden within is the downer story of a sad clown battling a fatal disease. The upshot: Adam Sandler is the star. The film’s sure to open at No. 1. How it holds on will be anyone’s guess.

1. Funny People: $35 million

While Virgin bowed to $21 million with an unknown Steve Carrell at the center and Knocked Up pulled in $30 million with newcomer Seth Rogen, Funny People has giant movie star Adam Sandler leading the charge. But the comedy pro is leaving his PG comfort zone and the results when he does so are never certain. Prognosticators have this film opening all over the map. I’m going with $35 million. Sandler and Apatow could prove to be a very formidable combination, even with a close to 2 1/2 hour run time.

2. G-Force: $17 million

Those covert guinea pigs proved to be a box office hit this last weekend at the movies. While Fox’s Aliens in the Attic will take away some of its power in round two, the movie’s unlikely to fall more than 50% for the frame. Pretty impressive that animated guinea pigs will cross $50 million after less than ten days of release.

Potter bows on IMAX theaters this frame, which will surely help cushion its drop in its third weekend. The movie has already grossed $221 million, more than its predecessor at the same time in its release pattern. If it maintains its momentum, Half-Blood Prince could hit $300 million by the end of its domestic run.

4. The Ugly Truth: $13.5

This Katherine Heigl-Gerard Butler update on the age-old conundrum of man/woman incompatibility surprised everyone last weekend with its opening take of $27.6 million. Turns out moviegoers like and want more romantic comedies, especially the R-rated ones. Expect a drop of around 50%, which would be quite a feat considering how critics hated the film.

5. Aliens in the Attic: $13 million

This PG-rated low-budget comedy originally titled They Came From Upstairs should open decently. From director John Schultz (The Honeymooners), the movie centers on a group of teenagers, including Ashley Tisdale, who must protect their vacation home from aliens residing in the attic. Kevin Nealon, Doris Roberts, and Tim Meadows co-star.