The PD 820
Better suited to most users' pockets than the pricey Pentium Extreme Edition will be a trio of dual-core CPUs without HyperThreading that all go by the Pentium D moniker. The bottom of the pile, so to speak, is the Pentium D 820, which runs at 2.8GHz. Next up is 830, pushing along at 3.0GHz, and then there's 840, running at an XE-matching 3.2GHz but, and it bears repeating, without HyperThreading.
Intel's 'Smithfield' line-up has two cores on a single die, each core with access to its own 1MB of L2 cache. But both cores share the same system bus, so memory bandwidth implications come more into play here than on a single-core setup. There are also the extra goodies that make it on to the Pentium 4 670, namely EIST, C1E and TM2 for power-saving, EM64T for 640bit processing, and Execute Disable Bit.
Intel's dumbed down the Pentium D line by withholding HyperThreading support. It's supposedly why the Extreme is extreme and the far cheaper Pentium D a not-so-extreme CPU. However, with two cores on a single CPU that can tackle a thread at the same time, HyperThreading won't be as missed as it would have been if absent on a single-core model, especially if you don't run more than a couple of CPU-intensive programs at any one time.
Intel hopes the clinching factor for the Pentium D is its attractive price. Ally that to a cheap-ish motherboard - say, any derivative of the 945-series - and it's dual-core action on the (relatively) cheap. AMD, on the other hand, is comfortable with the performance of its single-core Socket 939 CPUs, so the X2 range starts and finishes at the top-end of the market.
It's abundantly easy to see where the Pentium D 820 will succeed and fail, if judged by single-core models. Applications that are multi-threaded in nature will fly along, with two individual cores each executing concurrently. Think of professional apps and various media-encoding activities. Single-threaded applications will only be able to make use of part of each processor's ability so, for example, games won't be Pentium D 820's strongpoint. It'll run like a 2.8GHz 600-series CPU, if one existed. More importantly than ever, you need to evaluate your primary uses and then select the appropriate CPU. The Pentium D 820 has a TDP of around 95W. It hit around 58°c when under OS load and cooled by Intel's reference heatsink/fan combination.
A couple of things to note before we get on to the benchmark graphs. Firstly, the Pentium D 820's aggressive pricing puts it in line with AMD's single-core Athlon 64 3500+. Be sure to check out their relative performance. The Pentium 4 670, on the other hand, is slated to hit upon the current Athlon 64 FX-55's pricing territory. So it's pricey single-core against cheap dual-core.