While Dual processor setup may not offer the full potential for Photoshop or rather that Photoshop isn't able to take full advantage of todays computer resources ( is a real bloody shame!), but, most of us (I would hope) work on several programs when doing our post production tasks.
So, IMHO, a dual processor setup do has its place in the Digital Darkroom, when we multitask, such as perhaps using CaptureOne for your raw conversion, PTGui for your pano's and Photoshop for your refinement/final touch ups, all while you keep track of things here Lola :-) and you burn your disks for the client, and back up to external disks and all while you ready yourself for the next job.
I have, though now old in computer terms, two different setups, a Dual processor and a Single Processor setup, both have a total of 4 cores, and both have 8GB of ram. And both run at the same speed, 2.66Ghz. So all in all, pretty close (though the Dual is an Opteron, and the Single is an Intel Extreme Quadcore) if I benchmark them, there is 10% advantage to the Dual Processor.
However, the feel (non-tactile) that each of these computers give me is very different, I find it much more comfortable to work on my Dual processor system, it just seems "better", non of them crash, at least no more then once or twice ever year or so.
Some may know that I have started to make some noise regarding building another computer over the last few months, and trying to choose between AMD and INTEL, and with the help and insight here from Jack F and others, I am closing in on the Intel platform, mainly because it runs faster, over AMD's more cores but slower Ghz. So with that in mind, Lloyds findings are partly confirming the consensus here, but its also scary that our next new investments may not give us as much bang for our bucks.
Should one buy a dual cpu setup, but only buy one cpu, then upgrade that one with two, when Photoshop finally comes around with some better coding. You are still able to use more ram in a Dual CPU system, even if you do not have two CPU's in it, as the memory banks connected to the CPU can take more then 24Gb which I think is the max for single Processor computer setups (this goes for both Mac and Windows platforms).
So, I really hope Lloyd got it wrong, but I doubt it. ADOBE!!! We need better coding....and yes, there will be some that will say "...but some operations in PS can only be single threaded!" that may be true for now. :-)
If you really want to improve your systems performance, without having to go to the extra expense of buying a new both with new and super "slow" 6 core processor, spend your money on SSD drives, set up small RAID-0's (but remember to backup), it will help limit the major bottleneck in your system.
here is for hoping!