http://download-west.oracle.com/docs...iderations.htm

i quote

The hardware and operating system architecture on the primary and standby locations must be the same. For example, this means a Data Guard configuration with a primary database on a 32-bit Sun system must have a standby database that is configured on a 32-bit Sun system. Similarly, a primary database on a 64-bit HP-UX system must be configured with a standby database on a 64-bit HP-UX system, and a primary database on a 32-bit Linux on Intel system must be configured with a standby database on a 32-bit Linux on Intel system, and so forth.

(that took 2 mins to search)