Here's my two cents:
Karma Police - Radiohead [You know you want to]
Islands in the Stream - Kenny Rogers & Dolly Parton [One guy starts of singing, another joins in on the next verse; those two stop, another two start (following the pattern of Kenny; Kenny and Dolly) - then all 50 kick in for a rousing chant of "Islands in the stream, that is what we are...."
Talk Dirty To Me - Poison [Yummmm - "I gotta touch you cause baby we'll be....AT THE DRIVEEE IN, IN THE OLD MAN'S FOOORD, BEHIND THE BUSHES, TIL I'M SCREAMING FOR MORE - down the basement, lock the cellar door, and baby, talk dirty to me" - Hell, if that doesn't get you interested, then i've failed]
Don't Fear the Reaper - B.O.C.
Lovers in a Dangerous Time - Bruce Cockburn
Hounds of Love - Kate Bush/The Futureheads. [50 guys doing that whole "Oh-oh-Oh" has got to win you something...]
Young Folks - Peter, Bjorn and John [40 guys whistling, and 10 singing? Sounds like a plan]
pretty much anything by the Editors. Tom has a nice deep-ish voice. Van Morrisson? Weezer? The Who? All depends on what you want really...