Don't forget the Northwood is 3+ CPU generations back. Most people have moved onto Athlon-64 or C2D/Q. They don't have much going for them: single-core, small cache and run quite hot (but not as bad as a Prescott). Also most MB for the P4 have only PCI slots which is a back-end bottleneck (since it is arbitrated for use by a single device at a time and has a ceiling of 133Mbytes transfer bandwidth) and use less resilient non-solid capacitors. Also AGP graphics is a pain in the butt - since you get overcharged for newer cards and then you have the driver issues for these, etc.!!
You can't really charge that much for these CPUs these days!! They were only around the 200.00 USD mark (tops) when they were originally sold.
I am building up a machine around a Prescott for my sis's wedding (but that's another storey) - my 4Ghz project
!! Mind you I am using a hybrid server board with PCI-X slots... I wouldn't go back to PCI now that PCIe is well established!!
Lots of people use NewEgg (can't comment since I am stuck in the UK - with our high prices). I might even pull one off Ebay perhaps (lots for sale there)... But that would be a risk if someone has hard overclocked the CPU of course...
Bob