Hi Charles,
That post Mike links to is a great place to start your research on banking in Costa Rica. A lot of info on requirements to open an account and a rundown of the different banks available.
As far as rentals in gated communities with mostly expats, you’ll find plenty of those in Costa Rica. Start your search in the Central Valley (especially San Jose suburbs like Escazu and Santa Ana, as well as towns like Ciudad Colon and Atenas). You should also look at the Central Pacific coast towns of Jaco and Playa Herradura. Finally, north Pacific towns like Playas del Coco, Playa Flamingo, and Tamarindo. You’ll find a variety of homes, condos, and town houses.