Primary source for this information comes from Apple.com.
Requirements for Mac OS X v10.5
To use Mac OS X v10.5 Leopard, your Macintosh needs:
Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
512 MB of memory
9 GB of available disk space
Some features require a compatible Internet service provider; fees may apply.
Some features require Apple's .Mac service; fees apply.
Check out Apple Profile Specs for an overview of the supported hardware for Mac OS X.
Feature-specific requirements
Time Machine requires an additional hard drive (sold separately).
Photo Booth requires an iSight camera (built-in or external), USB video class (UVC) camera, or FireWire DV camcorder; and an Intel or PowerPC G5 processor. Backdrop effects require an Intel Core Duo or faster processor. Backdrop effects when using a DV camcorder require fixed focus, exposure, and white balance.
Boot Camp requires a Mac with an Intel processor and Windows XP Service Pack 2 or Windows Vista (sold separately).
Screen sharing in iChat and in the Finder requires a 128-Kbps Internet connection (300-Kbps recommended).
Front Row requires a Mac with built-in IR and an Apple Remote.
DVD Player requires a 1.6 GHz processor or faster for improved de-interlacing.
iChat requirements
Audio chats require a microphone and a 56-Kbps Internet connection.
Video chats require an iSight camera (built-in or external), USB video class (UVC) camera, or FireWire DV camcorder; and 128-Kbps upstream and downstream Internet connection.
Photo Booth and backdrop effects require an Intel Core Duo or faster processor.
Backdrop effects when using a DV camcorder require fixed focus, exposure, and white balance.
Some iChat features offer better performance and quality with higher system capabilities.
Developer tools require 1 GB of memory and an additional 3GB of available disk space.
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