An updated spec sheet reveals that the recently introduced Xeon E3-1290 runs at 3.6 GHz with four cores, but the chip's turbo boost will scale the clock speed to 4.0 GHz when running on only one core. What makes this processor particularly interesting is the fact that it is closely related to the i7-2000 Sandy Bridge series, which would indicate that Intel could be launching a 4 GHz desktop processor as well.
Intel pulled a single core 4 GHz processor back in October 2004 due to the increasingly high power consumption and current leakage in its 90 nm Netburst architecture. The highest clock speed reached by its Pentium family was 3.8 GHz, which was posted by its Pentium 4 HT 570/571 models, which were based on the 90 nm Prescott core. Back then Intel drastically changed its strategy from clock speed scaling to lower power processors that were largely based on technology introduced with the Banias Pentium M processor in 2003. That new strategy resulted in Intel's Core 2 Duo processors with Conroe core in 2006. With a look at power consumption, I should note that the Pentium 4 570 was rated at a thermal design power of 115 watts, while the E3-1290 runs at a maximum of 95 watts. The first dual-core processors, which were available up to 3.6 GHz (Pentium D 900 series), consumed as much as 130 watts.
There appear to be signs that both Intel and AMD are engaging in a careful gigahertz race again. AMD currently tops out at 3.7 GHz and we would expect its Zambezi desktop processors to easily reach 4 GHz later this year.