Although Mac OS X officially requires at least one of Apple's G3-based machines, there have been some recent development to work around this need. There now exist two untilties that allow Mac OS X to install and boot and run on so-called 'legacy' hardware.
The first is the free Unsupported UtilityX project by Ryan Rempel which is still in beta, is admittedly buggy but is totally free. It enables Power Macintosh models 7300, 7500, 7600, 8500, 8600, 9500 and 9600, as well the clones that were based on one of these systems (the Umax S900 and J700, and the Power Computing PowerWave and PowerTowerPro) to run Mac OS X using their original equipment (e.g. 604 processors) . It can be found at http://eshop.macsales.com/OSXCenter/.
The second option is by Sonnet Technologies and works in conjunction with their line of G3 and G4 processor upgrades. This is a much more 'polished' patch and has official support from a real company. However, it requires buying one of their processor upgrades and then paying 30 USD for the software. It only supports Power Macintoshes that use the so-called 'Processor Card' design, though Sonnet has pledged to make L2 Upgrades work as well. Sonnet can be found at http://www.sonnettech.com.
For all these options, though, I'm told there is still one issue that can't be overcome with software -- OS X still requires 128 megabytes of RAM.