DC...I'm going thru the same upgrade lust from my P4...
I've learned a lot from this and other boards...one factor to consider is the version of Photoshop your using...for example...my PS6 I'm told won't work with WIN Vista (so I'm stuck getting a machine running XP...not the worst to live with, but not so many new PCs out there still run XP), and I've read that only the latest CS# takes advantage of quad cores...
...Tim and others have said to go for the quad...me...I say match the hardware to the software...when I decide to go from PS6 to CS#, who knows what hardware and OS advances will be available for me...
Like you, I don't expect to put a heavy load on my machine so I'm sticking to a core 2 duo, an E5200 with 4gb ram, a custom built job that'll run XP and that'll let me install a couple of PATA 120/200gb hard drives lying around here.
...it'll run $450, less $100 or whatever I can get for my P4...a bargain upgrade...a cheap enuf thrill to ease the craving... 
My Canon kit 450D/s90; Canon lenses 18-55 IS, 70-210/3.5-4.5....Nikon kit: D610; 28-105/3.5-4.5, 75-300/4.5-5.6 AF, 50/1.8D Nikkors, Tamron 80-210; MF Nikkors: 50/2K, 50/1.4 AI-S, 50/1.8 SeriesE, 60/2.8 Micro Nikkor (AF locked), 85mm/1.8K-AI, 105/2.5 AIS/P.C, 135/2.8K/Q.C, 180/2.8 ED, 200/4Q/AIS, 300/4.5H-AI, ++ Tamron 70-210/3.8-4, Vivitar/Kiron 28/2, ser.1 70-210/3.5, ser.1 28-90; Vivitar/Komine and Samyang 28/2.8; 35mm Nikon F/FM/FE2, Rebel 2K...HTC RE UWA camera