From what I’ve read here and on tech forums, the dual-core chips seem to outperform quad-core because of memory bus issues. That said, as both apps and operating systems become more sophisticated at handling parallel processing, the quad-core is probably going to be a real boon.
If I could afford it, I would install a fast quad-core and the fastest possible memory. This should future-proof your new system as much as possible and should still run Photoshop like a scalded dog. And make sure the video card provides separate color LUTs for both DVI ports so you can calibrate & profile both.