Pentium Extreme Edition
On the regular Pentium 4 processors the difference between Pentium 4 and Pentium 4 Extreme Edition is the presence of a larger L2 memory cache on the later. But for the new dual-core processors (8xx series) the difference between the two is the presence of the Hyper-Threading technology, which is available only on Pentium Extreme Edition.
So, with a Pentium Extreme Edition in your system your operating system will recognize four CPUs, since each core will be recognized as two CPUs because of the Hyper-Threading technology.
So far just one Pentium Extreme Edition model was announced: Pentium Extreme Edition 840, running at 3.2 GHz and with 1 MB L2 memory cache for each core. As you can see, this CPU is exactly the same Pentium D 840 but with Hyper-Threading technology.