get a single core athlon 64 before you get a p4 or a pentium D.
theyre better for gaming... but id go with the core 2 duo if i were you. its definitely superior in performance per clock plus more overclockable than... yes... the pentium D 805... but only if you get like the e6300 and the gigabyte DS3 motherboard. or an asus should be good, just not the cheapest asus you can find.