I am not not an expert and not sure about total performance but I would go the Dual core route. I have a HP notebook with a P4 2.66 Ghz chip. It runs hot/loud and is a battery hog. I think the centrino chip is a better choice for a laptop.

Also I saw this today:

DellSB - Inspiron 9400 with 1.66Ghz Intel Core Duo T2300, 17" WXGA+ TFT, 512MB DDR2 SDRAM, 60GB HD, 24X CD-RW/DVD Combo, 802.11b/g Wireless for $899