After going over the needs for a good desktop PC, Ill walk through what you should have when youre on the go. In some ways, choosing a laptop for mobile photo editing is both more complex and simpler.
CPU and Memory
Right now, my production PC carries a Core i7-3930K CPU. The 3930K is a six-core CPU built on Intels 32nm manufacturing process, so it isn't using the latest-generation Ivy Bridge architecture. It costs about $560, and requires a motherboard that supports an LGA 2011 socket. Such motherboards can be pricey--particularly if youre into heavy-duty overclocking--but Im content to run the 3930K at its default settings, so in my system the CPU lives in a Gigabyte GA-X79-UD3 motherboard, which is available for under $240.
My machine also has 16GB of RAM, in the form of four Kingston HyperX LoVo DDR3-1600 modules. These modules can run at full 1600MHz (effective) speed at 1.5V. All of this helps the system consume less power. And that 16GB of RAM comes in handy when I have multiple raw images open: The D800's 14-bit raw files range from 45MB to 48MB each, but while I'm editing, they consume more memory than the disk space they occupy.
The 3930K CPU contains six cores, and can execute 12 simultaneous threads. That much power may be overkill for many users, so a good alternative is a CPU in the new Ivy Bridge line, such as the Core i7-3770K. Clocking in at 3.5GHz, the 3770K runs four cores and up to eight threads. Its capable, it costs around $350, and it's quite power efficient. You can buy a Core i7-3770 (note the lack of the K suffix) for a little less money. That version of the chip is not overclockable, but most users shouldnt care.
In addition to supporting Hyper-Threading, the Core i7 CPUs offer large L3 caches, which improve performance in most editing applications. If youre eyeing a laptop, ideally youll want a Core i7 CPU, mostly because i7 processors have larger caches than their Core i5 and i3 siblings.
As for the system's memory, if you edit lots of raw files, I still recommend 16GB of RAM, given that todays memory-module prices are pretty low.
Storage...and More Storage
If youre at all like me, youre shooting a ton of raw images. You may not be using a Nikon D800, but even 12-megapixel raw images consume at least 10MB of disk space each, while 12-bit, 16-megapixel images eat up 14MB to 16MB.