Many of the 20,000 in the overflow crowd drawn by the Jacksons and Sister Sledge to the Capital Centre Saturday night had been enticed earlier by non-ticket holders outside (5,000 were turned away) to sell their $8 general admission tickets for as much as $25. The show proved to be a bigger bargain.

The Jacksons mixed together medleys from their bubble-gum days, ballads in Michael Jackson's signature tenor, and selections from their new, 10th-anniversary album, written and produced entirely by The Five.

Their set started with "Dancing Machine," an older hit but still the perfeet introduction. The mechanical-doll dancing by Marlon, Michael, Jackie, and litter brother Randy, and the guitar-playing of Tito were exciting, crisp and precise - but not new.

Girls screamed at a spotlighted Michael Jackson's theatrical delivery of "Ben" and "I'll Be There." But "Destiny," the lyrically powerful title tune from the new album, which showcases their growth, was conspicuously omitted.

Concertgoers, who were on their feet until the show's close at 2:50 a.m., included packs of pre-teeners and fashionable middle-aged adults. But the majority of the audience was made up of older, black teen-agers; and promoter Tiger Flower and Co. used the opportunity and the presence of Stephanie Mills of "The Wiz" and Yolanda and Martin Luther King III (children of the slain civil rights leader) to urge the eligible to register to vote at booths outside the arena.

Sister Sledge, performing as a trio in the absence of the eldest sibling, shared lead vocals on all numbers including their current hits, "Greatest Dancer" and "We Are Family." The light/raspy, shrill/throaty vocal counterpoints of sisters Kim, Joni and Kathy made the group audibly interesting.