America Online on Monday announced plans to launch a Web-based radio service with XM Satellite Radio in a move that may help XM win customers from AOL"s 24 million subscribers, driving XM shares higher. The service, which will offer about 200 channels streamed over the Web in CD-quality sound for a monthly fee, will combine existing stations from both partners and will begin to roll out this summer. The service, which will cost an estimated $5 a month, will be available for free to AOL"s current paying subscribers.