Hyper-Threading Technology - 1 = 2
Hyper-Threading technology makes a single "physical" processor appear as two "Logical" processors under a Multi-processor operating system. To do this, there is one copy of the architecture state for each "logical" processor, and the "logical" processors share a single set of "physical" execution sources. In short this means that there are two architecture storage mediums but one actual layer of CPU. From a software perspective, this means operating systems and user programs can schedule processes or threads as they are known as, to each logical processor as they would on conventional physical processors, in short, the multi CPU OS's are none the wiser that there is actually only one single CPU.
Each logical processor maintains a complete set of architecture state. This architecture state consists of registers, including the general-purpose registers, the control registers, the advanced programmable interrupt controller (commonly known as APIC, yes that setting in the BIOS of some motherboards) and some machine state registers. From the software's point of view, once the architecture state is duplicated, the OS and other programs see 2 separate processors.
This implementation of Hyper-Threading technology only added around 5% to the die size of the already small Intel Pentium 4 CPU. Since the die itself is covered by the Integrated Heat Spreader (HIS) you will see no actual changes in the chip itself as compared to usual.