Using terms like "a new era begins," analysts are predicting healthy revenue gains for Mentor ( MNT) and Allergan ( AGN) now that they can sell silicone-gel implants in the U.S. for women who want breast enlargements. The ultimate improvement in sales and profits will depend not only on how many new implants the companies can sell but also on how many women choose the silicone version over those filled with saline -- the only implants approved domestically for cosmetic purposes for 14 years. Wall Street believes the number will be significant. On Friday, the FDA approved silicone-gel implants for cosmetic uses. Since 1992, the agency had allowed them only in cases of congenital breast deformities, reconstructive surgery following mastectomies or for replacing ruptured silicone implants. Analyst Peter Bye predicts the U.S. implant market will switch from 70% saline to 70% silicone "over the longer term." He doesn't give a precise timetable, but his recent report to clients at Wachovia Capital Markets says each 5%-market share switch to silicone from saline is worth an extra 1.5 cents a share to Allergan's earnings and another 5 cents for Mentor. Bye says the average selling price of a silicone implant will be more than twice that of a saline implant, but the manufacturing cost will only be 10% to 15% higher. He has outperform ratings on both companies. He doesn't own shares, and his firm doesn't have an investment banking relationship.