If you own an older PowerBook or iBook with a G4 processor, you should be able to install and use the latest version of Mac OS X (10.5 Leopard). Three years after abruptly announcing plans to ditch the PowerPC world and adopt Intel processors, though, Apple seems to have decided to banish non-x86 Macs from its next OS release.
As AppleInsider reports, a French Apple fan site has nabbed a PDF screenshot of system requirements for Mac OS X 10.6 Snow Leopard. The list mentions 512MB of RAM and 9-12GB of disk space—spartan demands for a recent OS—but starts with "An Intel processor" at the top. AppleInsider says the purported screenshot corroborates a report from last September, which quoted "people familiar with the matter" as speculating that OS X 10.6 would drop PPC support.
As we've already noted, OS X 10.6 will feature a number of new goodies, including a general-purpose GPU application programming interface dubbed OpenCL. Better multi-core support and a new version of QuickTime will be on the menu, too.