Hyperthreading technology in a 865 chipset family. Pentium 4 processors with speeds of 2.4, 2.6, and 2.8GHz. These new Pentium 4 "C" revision chips with Hyper-Threading technology and support for an 800MHz front-side bus, a bit slower by todays standard. They boosted clock-for-clock performance.
Very good deal for the price but no way they match the Core 2 Duo with much faster clock to clock speeds.
HT was Intel's implementation of SMT, or simultaneous multithreading, in which a single CPU presents itself to operating systems as multiple processors. Scheduling multiple threads at once allows the Pentium 4 to keep its relatively long 20-stage main execution pipeline full, increasing clock-for-clock performance.
Hyper-Threading does have its downsides, including some resource sharing issues.
For the skinny on the 800MHz front-side bus, you will want to read our review of the Pentium 4 3.0GHz and Intel 875 chipset. Those 865 chipsets Intel introduced big feature was an 800MHz bus... but that is now far from leading edge.
Those P4 "C" chips generally scaled better with clock speed, but not after Core 2 Duo came out. Just an older, but great technology but for the price, very good.