PowerPC 750FX and 750GX upgrades support a feature called "dual-PLLs". These processors have two software-configurable PLLs that control the speed at which the processor runs. This feature also works in conjunction with the Energy Saver control panel.
One PLL is configured as a slow speed to be used when the processor is idle. The other PLL is configured as the fast speed to be used when the processor is busy. This feature allows the processor speed to be automatically turned down when the processor is idle in order to save power and reduce energy consumption.
This results in a cooler-running processor, particularly important on notebook computers. In order to use this feature, you set the fast processor speed using CPU Director under the PLL tab (the slow speed is automatically set to be 500 MHz) to the speed at which your upgrade is rated. Then, you can select which speed the computer runs at by using the Energy Saver preferences panel. "Highest" corresponds to the setting selected in CPU Director, "Reduced" corresponds to the automatically-configured 500 MHz setting and "Automatic" will result in the slow setting being used when the computer is idle and the fast setting being used the rest of the time.
In short, due to it's dynamically changing CPU speed, be sure the CPU is running an application(s) that taxes the processor while you verify its speed. A quick way to do this is to use the Mac's Chess application. Set it to play itself on its strongest setting, and you should see the CPU jump up to its rated speed.
