Having been a photoshop user for years now, across MANY configurations, I can definitively offer the following experience:
1. 90% of the time, Photoshop needs high GHz CPU more than it needs high # of threads. More threads are always better if you can get them, but take care to maximize the CPU clock rate first.
2. Memory - RAM is so cheap right now, you are a FOOL if you don't max out your laptop. Prices only go up from here. Max it out with 8GB or 16GB if you can.
3. SSD - This is a must. SSD makes a HUGE difference in photoshop. Besides the usual opening / closing files, PSD relies heavily on the hard disk subsystem for what it calls "scratch disk." Essentially it stores / saves history states to the hard drive which can get HUGE. My own experience has been that switching to an SSD resulted in HUGE performance increase when working with large / complex photos or images.
4. GPU - Not relevant, as long as you are on the Intel HD 3000 generation. I have used Photoshop on both Intel IGP 3000 and ATI Radeon 6850, and I can say that GPU has virtually no impact unless doing 3D stuff. Then again who the heck does 3D with Photoshop??
Some comments in response to Voldenuit:
1. Good screen. IPS or high-gamut TN. Need color calibration if working with print media.
I don't disagree, but I will add to that a suggestion: If you're going to pay the price for a decent screen, you might as well get a Mac. They have the most consistently good screens available, even the Air's.
2. RAM. 6 GB minimum. That means 64-bit OS and Photoshop.
Make it 8GB. or 16GB. RAM is so cheap right now. Max it out.
Yes. Photoshop generally seems to favor higher clock rates. Only when you are doing certain filters will it run up more threads.
In terms of nice but not essential:
Strongly disagree based on my own upgrade experience. See above.
5. GPU. Contrary to some posters in this thread, I don't feel that a GPU is crucial to photoshop performance. The OpenGL acceleration in CS5 is minimal at best, and even very low powered discrete GPUs can handle them. IGPs are more than adequate for even power users at this stage.
Absolutely agreed. My own experience proves this.