The Pentium 4 65x runs at 3.4 GHz by default.
After enabling EIST (Enhanced Intel SpeedStep Technology), the 65-nm chip behaved exactly as the 90-nm version. If you want to save energy and reduce heat dissipation whenever the system is idle, switch the Windows power scheme from desktop to portable/notebook and verify the idle clock speed by using any CPU utility such as CPU-Z (freely available for download). As a result, Windows will order the CPU to reduce the clock speed to 2.8 GHz whenever possible and then automatically speeds it up again as soon as there is more work to be done.
SpeedStep turned out to be quite an effective way to reduce heat dissipation and to avoid unwanted fan noise. Although there is a slight impact on performance when SpeedStep is enabled, it is far too small to be noticed, which is why we recommend enabling SpeedStep for most users.
What is interesting, though, are the voltages the CPUs are running in full power and SpeedStep modes. While our 90 nm Pentium 4 660 Prescott goes from 1.35 V to 1.15 V in SpeedStep, the Cedar Mill Sample would go from 1.35 V to 1.30 V only. However, as this is an early qualification processor sample only, we cannot make any statements about the core voltage(s) of the final product.
After switching SpeedStep on, the system will automatically switch from 3.4 to 2.8 GHz whenever the CPU workload is small.