For an understanding of Intel CPU architecture, see Intel microarchitectures. For Pentium 4 and Celeron D in LGA775 chip package ... Similar to 820, but adds support for 64-bit PCI, 4GB of ...

they both share the Intel 64 64-bit technology, but the Pentium D does not include Hyper-Threading. The Pentium D's two execution cores provide two completely parallel processing streams.