p4 probably would have ht(hyper threading--acts like a dual core, sort of, but is only one chip(great for multi-tasking))
pD is dual core(runs both cores at a lower speed to improve efficiency and keep it cooler)
IMO if you want to go dual core, get a AMD atholon XP2 or pentium Extreme Edition(EE). the pD is great, but the others are more powerful
i would go dual core also, not HT
thats it