For Mac OS X, based upon BSD, Apple had to make a decision as to how the GUI would work. They could, on the one hand, follow the traditional UNIX path of using X11 to display data; or, they could create an entirely new windowing system. Apple chose the latter option, developing Quartz: a proprietary graphics layer which is itself based upon NeXT's Display Postscript, this time around using PDF to draw 2D graphics. This is happily supported by any Mac which can run Mac OS X, ensuring the Aqua user interface is consistent across the platform.
For the third major release of the operating system, Mac OS X 10.2 (Jaguar), Apple extended Quartz, developing 'Quartz Extreme'. This uses OpenGL to render graphics much faster than Quartz alone, by rendering two-dimensional images as textures on a 3D OpenGL polygon. Modern 3D accelerators are more than capable of providing speedy OpenGL performance.
Apple decided to restrict Quartz Extreme only to hardware which would work effectively with it. The minimum specifications are a graphics card with either an ATi Radeon or nVIDIA GeForce 2 with 16mb of VRAM, running over the AGP bus.
AGP is considerably faster than the older PCI bus, which itself was given a lease of life with the 66Mhz ports available on more recent Power Macs. However, many still ran video cards over PCI; the Mac version of the Radeon 7000, in particular, is a popular choice of video card for the older Mac. They would easily support Quartz Extreme if it weren't for the AGP restriction.
Enter PCI Extreme. This is a small program written by Zack Schilling, which will enable PCI Macs with a Quartz Extreme-capable card to use it. PCI Extreme will only work with a compatible PCI video card - you cannot, for example, enable it on an aging Rage II - but, if you have one, it offers a performance boost for general usage of 5-20%. (This does come at a cost. Due to running the bus-intensive Quartz Extreme over PCI, any other devices on the PCI bus will be considerably less responsive.)
Simply download the program from http://pages.cthome.net/zacks/index2.html, and run it. You will then be offered the choice: activate Quartz Extreme on the PCI bus, or over the AGP bus (i.e., disable PCI Extreme). The program will need your password in order to perform some Terminal wizardry, then ask you to restart your Mac. Once it's booted, you should be able to confirm Quartz Extreme is running by going into the System Profiler (click the Apple menu, then About This Mac, then More Info) and choosing Graphics/Displays.
PCI Extreme is not in any way, shape, or form supported by Apple, and you use at your own risk. There have been reports of system instability after using it, which is a risk whenever running a tweak such as this. It is also possible to choose the wrong bus to enable Quartz Extreme on, thus rendering your Mac unusable. To fix this, boot up into Safe Mode (hold Shift immediately after the chime when booting your Mac, then release once the Apple logo appears), run PCI Extreme, and choose the other bus.