no you can run regular 32bit windows xp on a 64 bit processor. there is also a 64bit windows xp pro just for 64bit processors that you can get, however, that has many unsupported drivers, so for the everyday user, its better to just get regular 32 bit windows.
when he said if you know what you're doing, i don't think he meant overclocking. i think there are strategies to getting the most out of your dual core.. i think you can set a few things for one core, and the main thing that you're doing on a seperate core. i don't have dual core so i don't really know.. but i think somethin glike that.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius