backup/restore in case a fatal server fa

Gentlemen, a customer of mine want us to implement a plan for backup/restore in case of fatal failure (system down) of the servers.
There are four critical servers runing in NT 4.0, UNIX and Linux, one of them with the ORACLE database, another witha critical telephone aplication. Can anyone help me telling where can I find some information about ?
Your assistance will be greatly appreciated.
Regars: Y. Solano