"Baby Boom," the Diane Keaton comedy, debuted at No. 16 on the Billboard magazine rental chart and could be in the Top 10 next week. A pretty strong box-office performer, this one has Top Five video potential--along with "Adventures in Babysitting," which is already in the No. 3 perch. The thriller "Someone to Watch Over Me" is picking up steam, moving up seven notches to No. 8 on the rental chart. The Cheech Marin comedy, "Born in East L.A.," is a surprisingly popular rental, barreling up to No. 15 in two weeks. "Weeds," starring Nick Nolte, is off to a good start, climbing quickly to No. 21. "The Whales of August," with Bette Davis and Lillian Gish, didn't make a big splash in the rental market; after one week at No. 37, it sank out of the Top 40.