If you need to install Mac Os X Leopard on your PowerPC that does not meet all the requirements, you can use this leopard assist tool to temporarily fool the system. You still need your own 10.5 Install Disc.
Download the tool here: LeopardAssist.app
Perhaps LeopardAssist looks a little basic on the surface. A few clicks of the mouse is all LeopardAssist needs to configure your machine for installation, from the processor to the drives – all without requiring any manual editing of files or burning of additional discs.
LeopardAssist has several years of testing to its name, across most models and series of PowerPC Mac, and even today it continues to receive feedback and feature requests for consideration. In addition, developments and information gathered in the improvement of LeopardAssist have assisted in helping other Mac related projects on both PowerPC and Intel.
LeopardAssist is widely used and supported across many specialist Mac communities, such as the ThinkClassic Vintage Apple community ( thinkclassic.org ) and the MacRumors PowerPC Macs Forum ( forums.macrumors.com ). Any questions related to the use, care and maintenance of your PowerPC based Mac and the installation of OS X Leopard can be directed to one of these communities.
Source – http://leopardassist.sourceforge.net