Apple has published the system requirements for 10.5 Leopard. They’ve cut the PowerPC G3, and any G4 lower than 867 MHz from the supported macs. All PowerPC G5 and Intel macs are fully supported in 10.5 Leopard.
The OS takes 9 GB and the developer tools take an extra 3 GB (totaling 12 GB). What we need to remember is that actually it won’t be 12 GB from what our macs are now, because the space that Tiger is taking up (14.43 GB; System (2.29 GB) + Library (12.14 GB) folders) and the 1.8 GB that the developer tools currently take up gets counted into the total for the new OS. Enjoy, Alex.