Backup for Web Hosting Servers

We are Web application Developers and we are planning to setup our owns server to offer hosting to our clients.
At first we have planned to install 3 servers at the datacenter: 1 for the web server and the application server, 2nd one for SQL serverand databases, and the last one for a mail server.
At the moment we need to choice a backup politic for this infraestructure. What is the best one for our small datacenter?