I got this from the German site Apfelgeruechte.de.
It is possible to activate Quartz Extreme graphics acceleration for Macs with PCI graphics! This is, of course, espacially useful if you upgraded your Mac with a modern 3D graphics card like the ATI Radeon, as I did in my old B&W G3 350.
All you have to do is alter a setting in a plist file. Make sure you have administrator privileges and open: /System -> Library -> Frameworks -> ApplicationServices.framework -> Versions -> A -> Frameworks -> CoreGraphics.framework -> Versions -> A -> Resources -> Configuration.plist. There you will find an entry named GLCompositorConfiguration - GLCompositorRequiredClass with a value of "IOAGPDevice".
You may either replace this with "IOPCIDevice", or add a second entry to the GLCompositorRequiredClass array. Save the plist, restart your Mac, and check Quartz Extreme by turning on zoom in the Universal Access preferences. Now it's feasible even for my G3 350 to run an OpenGL screensaver as desktop background! :-)
[Editor's note: Has anyone done any pre- and post-hack benchmarks on a PCI machine to see the actual improvement in the graphics display? I'd be curious to know how well this works...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020830061842401