CORE 2 DUO Intel Processors

Intel CORE 2 DUO are new generation completely redesigned in 65nm while the D series are updates of old generations and its 90nm. core 2 will make less heat which give a little advantage in performance already because of the 65nm. core 2 use less voltage which mean the quality of the circuit is better and make less heat. they incorporates more technologies like EIST and other good stuff i dont remember. core 2 integrated more cache in the cpu wich greatly improve ther performance.Intel Processor

core 2 E6600 and up are all better than the D series. dont look the speed in Ghz. its just misleading. the lower speed new generation design is more powerfull than the older one.
they just out perform anything at the moment (that includes the AMD AM2 Athlon64 and Athlon FX series)

If you are building a new computer. IF you have money take the core 2 extreme but E6600 and e6700 are already very good.
take a nvidia video card like 7600gt, or 7900gt, 7900gtx or 7950gt(x).

if you build a pc, use ASUS for best performance and reliability. They are the world leading software and hardware computer builders. Very enovative and design in each of their components compared to their competitors. For best performant and inovative motherborad look at the boards that are colored in black with either the latest nidivia or intel chipset. if you are a gamer i think you will take nvidia, but both are good. Those black boards offer the best features and new technologies like Dolby ditial HD audio, Hyperpath (increase memory efficiency), AI booster or AI gear (instantaneousy overclocking without restarting pc or going in the bios), etc. Asus give tones of free softwares which are expensive enough if you buy it yourself.