why the price difference between Pentium® 4 Processor 640 w/HT Technology and Pentium® D Processor 820

The prices below are from Dell's website and the model is the Dimension E510 Series.  From what I have read, the Pentium D dual core is better than the P4 with HT technology yet the prices for the Pentium D is much less.  What is that?  If it is clock speed then I wonder why Dell won't allow you to configure higher processors and only give you the choice of these 3 on their media additions.

Pentium® D Processor 820 with Dual Core Technology (2.80GHz, 800FSB) [Included in Price]
Intel® Pentium® 4 Processor 640 w/HT Technology (3.2GHz,800FSB) [add $50 or $2/month2]            
Intel® Pentium® 4 Processor 650 w/HT Technology (3.4GHz,800FSB) [add $110 or $4/month2]
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Gary CaseRetiredCommented:
As Callandor noted, it very much depends on your use of the PC.   The single-core options you've listed not only have higher clock speeds, but also have twice the cache of the individual cores of a dual-core CPU  (Both the 640 & 650 have 2mb of cache;  the 820 has 1mb for each core -- the same 2mb total, but not as efficient for an individual thread).   Software that does not take advantage of the dual cores will run better on a single core CPU.

Dell's pricing does not necessarily reflect the actual market values of the individual processors;  it is simply what they have set as their own price points.   In fact, a 640 is currently LESS than an 820 at Newegg ($214 for a 640, $219 for an 820).   (a 650 is $270)
It depends on what you use the computer for.  If you multitask or use applications that are multithreaded, a dual core is the preferred solution.  If you play games or use applications that are single threaded (which means most office applications), a faster single core is better.  Dell is a common denominator provider, so I wouldn't expect them to allow you to customize so much that you would be able to get exactly what you want.
Gary CaseRetiredCommented:
It is interesting that Dell doesn't offer any of the higher-speed Pentium-D's with their Media Edition PC's ==> they offer all of the Pentium-D's up through the Extreme Edition 955 with the XPS systems.   I suspect that's a simple marketing decision on their part -- there's no technical reason for this limitation (with the possible except of the Extreme Edition, which may not be supported by the BIOS).

Gary CaseRetiredCommented:
bd1saul  -- what did you decide to buy??   Did you have any follow-up questions?
bd1saulAuthor Commented:
Thanks for the help and I apologize for taking so long to respond.  Basically to answer garycase's question, we made two decisions on computers to buy.  My girlfriend went with the pentium D dual core while after showing the information I got from this site, I kind of stayed out of it.  However from this and another question that I had asked, I am going with a Monarch for my next computer.  
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