'Photoshop' the app is not so multuthreaded but it will recognize multiple cores - it's your 'typical' filters and plugins you need to study up on and see what benefit there is to gain.
A nice OpenGL card for your 'Zoom & Pan', a fast dual core CPU, 64-bit OS, 6Gb of RAMs and a fast capture/scratch drive will take you a long way.
In a lot of cases a higher clock speed vs more cores (with the same processor architecture) will win the 'race'. That's where an e8400 or Phenom 720BE at 3.6GHz will do yah just dandy. BUT as Wuzy noted some of the plugins (s)he uses will take advantage of 4 independent threads but it is truly 'buyer beware'. Don't assume because you have 2 times 2 cores that certain operations will be twice as fast.
Lightroom for the most part is memory and I/O bound. It actually depends in large part about the level of your operations.
It's also my understanding the Lightroom on a PC is far superior to that on a Mac (Don't be mad - I'm just passing on what I read )
Why did you pick the Q8200? I have a Q9400 and it seems to run Photoshop Elements real well. My Q9400 was able to handle a big overclock, and it cost the same as an E8500, so essentially I got the extra 2 cores for free.
If you really want a fast machine you might do some pricing and determine if you can afford an i7.
I am not sure about the latest version of Photoshop, but Photoshop CS3 doesn't deal
with multiple cores at all according to this article "http://www.tomshardware.com/reviews/multi-core-cpu,2280...". In addition, as long as you are not an extreme gamer, even a single core CPU with a good graphic card, such as 8800GT, will be more than sufficient! In my opinion, it will be fine to stack with your single core CPU for at least a year and then upgrade to a quad core in the middle of 2010.
The quad core will be much cheaper and more common then, and I believe that more applications will be much well designed for the support of quad core CPUs after this 1 year buffer. That's what I am going to do.
Since CS3 I've been using Topaz Adjust w/ DeNoise and Nik's filter in PS a lot. (Using CS4 now) Those filters demand multi-core. I don't use the built-in PS filters much, which I think most are still single-threaded.
Photoshop CS4 requires only a card that support OpenGL2.0 for GUI blings. Even integrated like G45's X4500HD will do.
I am using (actually my wife) CS4, but mainly Lightroom 2. As for the operating system - we'll go with XP Pro 32-bit for now for compatibility reasons (the machine is also for casual and everyday use) so 4GB of RAM will do for now.
wisecracker - I read you on the speed vs. cores argument, so let me elaborate: PS is used rather seldom, while Lightroom is the main app. Moreover, my wife shoots mostly events and weddings, so we're talking about large batch operations on Raw images - specifically, import AND export, as well as building all the library previews (~1500 images at a time) and parameter changes on many images at once.
This is why I opted for more cores. To my understanding this is where I should be seeing the performance gain.
As for the Mac vs. PC performance - I don't use LR myself too often, and anyway I have a dual quad-core Intel Xeon 2.8 Mac Pro, so it won't be noticeable anyway