I also would say go with the quad, either the 6600 or 6700. Really since you want as much future proofing as possible the quad is the way to go. As more and more software is designed for multi-cores you will get more and more out of the quad. And as was said Photoshop is fairly multi-core friendly.
And again as others said don't worry about the PCI-E 2.0, we just don't use all of 1.1 yet. As for the video card I would say keep what you have right now until the newer GT200's (9900's?) come out. Better off to wait just that little bit and be able to get something much better than available now.
That's my 2 cents.
"Nothing sucks more than that moment during an argument when you realize you're wrong."
I keep flip floping between the Q6600 or even a Q6700 as they are almost the same price now vs. a 8400. Again I'm battling with future software and games. I think either cpu will make me happy to be honest.
I need to stop worrying so much about the small details and just find a good deal on decent parts. Having lived this long on a amd64 3700 should teach me that much
Most definitely. But having used both chips (Q6600 and E8400) with CS3, I can tell you there was a noticeable diference with the Quad.