For my type of work I would stay with 4 cores. PS CS3 and Aperture. http://www.barefeats.com/octopro1.html shows no gains with my programs on an 8 core machine.
Take a look at those benchmarks and see what might be better for you.
I love my 3GHz 4 cores but I would be as happy with a 2.66. I don't know in numbers how much faster the 3 is compared to the 2.66 but my butt dyno says it ain't worth $800. That money will be better spent in X1900XT and RAM.
One Thing that I need to mention, is I am about to switch from a powerful PC, 3.73 GHz Dual processor Intel Extreme Edition with 3.25 GB ram, running XP Pro with 4 each 500GB Hard drives, Diamond x1600 ati video card with 512 MB ram and 2 RW dvd drives. So I am a bit worried that if I don't go the 3.0 Ghz route instead of the 2.66 GHz route, that I may actually see a drop in performance but I could be wrong. What do you think?