A few things. As a couple have already mentioned Casey from Signburst can really help you determine what will best fit your needs. Things will largely be dictated by your budget. I'm of the thought of future proofing as much as possible which does also equate to spending more now to be sure the system will be up to the task for as long as possible. I'm also all about business/commercial class over consumer class components. Consumer and enthusiast class hardware is more geared towards lower budgets/gaming/less frequent use at the possible expense of stability and long term reliability. There have been numerous threads on this in the past though so rehashing it all again is not something I'm going to do other than a few key points. Multiple processing cores - most important for multitasking and applications that can actually take advantage of it like many of Photoshop's functions and filters. High end video card - sure if you are using the core of your business for playing games then that will be important. You are likely designing mostly 2D although one important thing is that current versions of Photoshop (above CS4 I think) can and will take advantage of a decent video card for some things which can really help Photoshop run a lot smoother and process certain filters and functions much faster. Ram, cmon', its not terribly expensive - the more the merrier. 8Gb is really coming to be the minimum anymore (assuming you are planning on a 64 bit operating system and applications where possible otherwise anything more than 4gb is pointless). Photoshop has been 64 bit for quite a long time, Illustrator as of CS6 as well. 8GB might seem like a lot but load up a 1GB file in Photoshop and make just a few changes (each change has an undo state that will gobble up your RAM very fast) and you have already used all ram and are having to swap data to disk. An SSD is a near absolute must and yes that will make the Photoshop swap file a ton faster but an SSD is still nowhere remotely as fast as RAM is and the more you can have your working data in RAM the faster things will be. 8GB is standard on many consumer class desktops now with business class Professional Workstations having capabilities of 32Gb all the way up to 512GB and more of RAM now. Again, an SSD is nearly an essential must nowadays. A good 250GB SSD can now be has for well under $150. Make that your boot/programs/swap drive and get a nice big regular hard drive for bulk data storage. Even better is get a second smaller 80 to 128 GB SSD dedicated as your "swap" drive then the big mechanical drive for bulk data.
Regardless - get a business class system from Signburst, Dell, HP. Professional Workstations and business class systems often sacrifice a tiny (truly negligible) bit of performance for bullet proof day to day reliability. Business class systems generally are also not pre-loaded with bloat-ware useless extra software.
