Sylvester Stallone's Rocky Balboa came out swinging on Wednesday, pummeling the competition with a take of 6.4 million in its debut, but the real contender for the box-office championship over the holiday weekend doesn't even enter the ring until tonight (Friday), when the Ben Stiller comedy Night at the Museum opens. Two other potent challengers also enter the fray: We Are Marshall, based on events surrounding the 1970 plane crash that killed the Marshall University football team, and The Good Shepherd., the Robert De Niro film about the early days of the CIA, while Dreamgirls, about a Supremes-like female pop group expands to 852 theaters. Most box-office analysts are betting on Museum to win with about $35-40 million, followed close behind by the latest Rocky sequel, which, they say, is due to take in about $30 million.