The overland trip is intresting too. Mundo Maya and Linea Dorada both leave from the Marine Terminal/Tourism Village area several times daily for Flores. If you want to save money, this is the way to go. The shuttle bus ride from Belize City to Flores is $20, and you can get nice accommodations in Flores for well under $50 U.S. a night. Transportation from FLores to Tikal can be found easily with travel agents right on the street. This route takes 3 days though. The shuttle bus ride from BC to Flores is 5 hours.
--Lee