Snow Leopard is 10.6, and as mendel says, it can't be installed on any PowerPC Mac. 10.5.8 Leopard is the newest system that an iBook G4 can take, and it does not support Classic. This means if you upgrade past 10.4.11, either using a Firewire xternal hard drive with 10.5 or later, or partitioning your hard drive is recommended if you have any Mac OS 9 software you still depend on. Partitioning erases the hard drive. PowerPC Macs can only boot Mac OS X off Firewire when it comes to external hard drive. A few exceptions are out there, but are difficult to locate. G4 867 Mhz or higher or G5 is necessary for 10.5.8. Pre-August 28, 2009 Intel Macs can also run 10.5, and have an added capability to use Boot Camp, to run Windows.