Apple states the following basic Leopard system requirements, although, for some specific applications and actions (such as iChat backdrops) an Intel processor is required:
Processor must be any Intel, PowerPC G5 or G4 (867 MHz and faster)
DVD drive (for installation of the operating system)
At least 512 MB of RAM (additional RAM (1 GB) is recommended for development purposes)
At least 9 GB of disk space available.