Anyone who came of age in the '70s surely has a nostalgic bond with the gangly showman-nerd of bad fashion. In my case, my older sister is a huge (but discriminating) fan. It's easy to hear Barry Manilow's knack for stick-in-the-cranium hooks, average-guy sentiments, and production tricks. There's a reason we all remember ''Weekend in New England'' and ''Mandy.'' Still, the four-CD Complete Collection And Then Some is expressly designed for cultists, with its alternate-take versions of his hits and for-fans-only rarities. Compiled with evident care (Velcro-attached fold-out sleeves plus an hour-long video), the box shrewdly gets around his pop-chart decline in the last decade by sequencing the 70 songs out of chronological order. Still, at the risk of infuriating my sister, this overdose of Manilow will be, for most, a C.