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]