For a certain group of aspiring actors, dancers and singers, Tampa has become an essential step on the road to Broadway. Every year, some of the most promising students with serious aspirations for stage stardom study with the Broadway Theatre Project, an intensive three-week program founded by Tony winners Ann Reinking and Debra McWaters. Usually about 180 students attend. This year 200 students from the United States and Canada (and three from Samoa) were accepted. The program culminates in a revue titled "Musicals . . . The Old and the New'' Saturday at the Tampa Bay Performing Arts Center. BTP students will perform scenes from all eras of musical theater, including selections from an upcoming show by Frank Wildhorn. The annual BTP shows - this is the 17th - are often-dazzling showcases that give one cause for optimism about the American theater's future. Performances are at 3 and 8 p.m. Saturday at TBPAC's Ferguson Hall. $35 plus service charge. (813) 229-7827 or tbpac.org.