The 'SYSmark Performance per Watt' can be misleading when comparing the dual and quad core processors, particularly when looking at other types of loads that make better use of the additional cores. In particular, look at x264 encoding, where a quad-core processor offers nearly double performance. See the graysky's articles on TechARP for actual charts and tables:
http://www.techarp.com/showarticle.aspx?artno=442
http://www.techarp.com/showarticle.aspx?artno=520
For a load that will use all processors, I believe that testing will show better performance per watt for a quad core compared to a dual (or even single) core because there is little or no additional overhead from the motherboard or powersupply or hard drives when adding an additional core.
It may be easier to understand this by comparing running 2 separate PCs with dual core CPUs in them vs. running a single PC with a quad core CPU. The quad core system won't need another motherboard, power supply, fans, drives, etc. so all those items are areas of power savings.
From the published tests above, the 2 extra cores consume 195-132=63 watts at max load and 94-77=17 watts at idle. That is in contrast to adding a 2nd system identical to the E6850 testbox which used 132 watts at max and 77 at idle.
A simplistic scaling using a spreadsheet, and assuming that performance doubles going from dual to quad cores (reasonable for x264 encodes), shows that:
In other words, using the system power numbers given, the quad core can be 35% to 64% more efficient than using a dual core, if given an appropriate load. A look at the Sysmark benchmark scores will show you that it did not scale up very much going from dual to quad.
You said, "You cannot upgrade an existing Pentium D or Pentium 4 system with a Core 2 Duo processor, so you will have to purchase a new motherboard supporting the new processors, as well as DDR2 or DDR3 memory." My wife's P4d is on a Fox 45cmx mobo. The E6850 is listed on the support list. I was thinking of upgrading to the E6850 dual core.
