Okay, right now you'd be better off getting a Mac Pro, dual dual-core Xeons, and if you were to build a similar setup yourself it would cost like $50 less than just buying a Mac Pro, and if you get a Mac Pro you have the option of using Mac OS X-specific image editing software like Aperture (which rocks for raw imaging).
And dual-core is two processors in one, dual processor is two independant processors. The only difference is that dual socket motherboards are extremely expensive and i think dual processor setups use more electricity.