With AMD's new release of its dual core line it's time to start the frenzy of reviews and comparisions.
Here is my 30 second summary:
Basically just two Pentiums bolted together in the same package. They share the same 800 Mhz front side bus. All interprocess communications goes through this 6.4 GB/s bus. Due to heating concerns both procs run at lower clock speeds than the current single proc pentiums. The D requires new motherboards and chipsets, but is priced almost half of the X2 price.
The core was designed from the beginning to be multi-proc. The architecture communicates between the two procs via hypertransport that when combined with the memory bandwidth yields 14.4 GB/s of total processor input/output bandwidth. The procs are available at the same clock speed as their single proc counterparts. The X2 does not require a new MB, you can drop it in your 939 slot with a bios upgrade, but is is nearly double the PD price.
Best to read the reviews and draw your own conclusions.