I hope you are aware of the 'can of worms' that you've just opened. Few things will get an internet forum fired up like a PC vs Mac debate.

I'm a PC user, but if I wasn't so concerned about the price, I'd probably go for a Mac.
First thing...why are you considering a laptop for Photoshop? There are obviously plenty of advantages with a laptop, but there are some some negative points as well.
When editing photos, it can be fairly important that your editing be consistent...and that can be challenging if your view of the display changes often. Besides the fact that the screen will be opened to different angles, you may also tend to use it in different locations...each with their own unique ambient lighting. This can mean that you see the display differently, and thus you would edit the photos differently.
Most laptops have a TN (twisted numatic) type display...these are less expensive, but they will look quite different depending on the viewing angle. The better type of display is IPS (In Plane Switching), but they are more expensive. If you are set on getting a laptop, look for IPS display.
But really, the better option (if you must use a laptop) is to set up an external 'desk top' monitor and workspace where you can dock your laptop when you're doing critical adjustments to exposure & color.
Either way, you should factor in the costs of a display calibration system.
As for hardware, I've always heard that the video card isn't nearly as important for editing still photos, as it is for gaming or video. So don't splurge on a tricked out video card, but do make sure to get a nice fast processor and plenty of RAM.