Most Intel Pentium D buyers are going to buy Pentium D 820, not the "extreme" 840. Pentium D 820 costs US$270 even in South Korea where AMD Athlon 64 X2 at the lowest clock costs over US$600. So comparing Pentium D with Athlon 64 X2 isn't proper. Athlon 64 was developed for "next-generation" CPU market while Pentium/Pentium 4 is quite old and perhaps should have been replaced years ago. Though Intel has made a lot of money in the recent few years from Celerons and Pentium 4, both Itanium and Pentium 4 Netburst series seem to be Intel's biggest failures yet. Pentium M and Tualatin have been good though not available on the widest platforms.
Most cost-effective AMD CPUs for the average consumers look like Venice at its lowest clock speed. Most cost-effective Intel CPUs are either Celeron D or Pentium D. Celeron D is very cheap and very overclockable but has very small cache memory and has only one core. Pentium D has dual cores and 2MB cache memory but costs at least 2x more than Celeron D. Celeron D, Venice, Pentium D, Athlon 64 X2, Xeon... they are all for different markets. Intel should accelerate their release of next multi-core processors based on something even better than Pentium M Dothan.