machan there are no two such thing as pentium D & dual core. They are the same processor.
Intel pentium series
Intel pentium 4 - Ordinary 32bit CPU.
Intel pentium 4 HT - same as pentium 4 but supports hyper threading. todays programs run on threads. just think of it as a light weight process. more threads mean fast
Intel pentium D - this is same as pentium 4 HT. but a single physical core is seperated into 2 cores using softwares (firmware)the processor actually work as 2 seperate cpus, enethought physically only 1 core is there... the performance gain is like 30-40%
ex: pentium D 2.8 GHz will work similar as 2.8 x 1.4 GHz (approx)
Intel Core series
Intel core 2 duo - It has 2 physical cores. no firmware tricks or anything speed is doubled. core 2 duo 2.6 GHz will give a performance of 2.6 x 2 GHz ...
But to gain
max out of it U should use softwares that use the max out of this technology like play good games

or use parrellel processing applications & so...
Hope you get the idea..
Cheers,