MB,
Following are Tiger's requirements. No doubt Leopard's will be the same or even a bit more rigorous.
Requirements:
• Macintosh computer with a PowerPC G3, G4 or G5 processor
• Built-in FireWire DVD drive for installation
• 256MB of RAM
• 3GB of available hard disk space (4GB if you install the developer tools)
• Processor upgrade cards are not supported. View a list of compatible hardware
Here• Some features require a compatible Internet service provider; fees may apply.
• Video conferencing with iChat requires a compatible FireWire DV camera or web camera and broadband Internet access. One-to-one video conferencing requires a G5, G4 or 600MHz or faster G3 processor and 100-Kbps or faster broadband access. Initiating a multiway video conference requires a G5 or dual 1GHz or faster G4 processor and 384-Kbps or faster broadband access. Participating in a multiway video conference and initiating a six-person multiway audio conference both require a G5, 1GHz G4 or dual 800MHz or faster G4 processor and 100-Kbps or faster broadband access.
• .Mac sync requires a .Mac account; fees apply.
HTH