I've had the chance to meet and talk with Casey before and I can tell you he really does know his computer systems well, and they are great machines for design work.
To output to a single printer, most mid-range desktops today would offer the capabilities of doing this for you without much difficulty. However, there are very specific components and combinations that can make a system more than just "capable" of the job and actually handle it with incredible speed and efficiency. Depending upon how much designing you are intending to do on the computer (and the level of responsiveness and demand you are placing on it in Adobe) then you may be needing some more performance benefits in other areas as well.
For design work and output to a printer/plotter, there is very little GPU intensive work that is done. In fact besides helping to speed up some renders in Photoshop, the only benefit from more powerful graphics cards that you may get would be smoother scrolling while in massive art files, higher resolutions, or in the case of professional desktop graphics cards, better color fidelity. Instead, the majority of your computation is going to be placed on three things in this order: Processor, Memory, and Hard Drive.
Get a decent processor, at least a quad core, preferably with hyperthreading, like an Intel Core i7 series (Ivy Bridge or Sandy Bridge-E) to leverage the maximum multitasking capabilities when your computer really needs to crunch some numbers. Going with an Ivy Bridge platform computer is going to be cheaper overall, but if you want the absolute greatest performance and want to support a huge amount of RAM, then step up to the LGA2011 Sandy Bridge-E platform.
Memory is cheap right now, so go ahead and load up on as much as you can afford. Most desktop users are never going to really need more than 4 GB for daily usage, but when you're designing and outputting large graphics files, the more RAM you can throw at it the better, especially in programs like Adobe that can utilize all that extra memory. There is a delicate balance between high speed memory (for example, DDR3 1600 Mhz) and low timings (for example, CAS 8-8-8-24). This is getting pretty detailed, but if you are having a custom system built this may be something you want to have specified or clarified for you is the memory type, speed, and timings. You will get higher throughput and thus greater performance from a system with higher speed and lower timings! If the system is an Ivy Bridge platform, this has a dual-channel memory controller which means for the best performance you should use pairs of memory sticks. Likewise, the Sandy Bridge-E platform is a quad-channel memory controller so the best performance is going to be in groups of four memory sticks.
Hard drive recommendations are going to vary greatly depending upon how you use your computer and the rest of your computer network. For example, are you saving artwork files directly to the computer or do you have some form of network shared storage? Ideally, storage of files should be moved off the local machines onto a redundant shared storage device, meaning all your RIP and design computer has to do is RIP and design, not file sharing and backup as well, and you don't have to worry about losing files if your computer goes down. If you don't have to save a bunch of files to your local computer, then you don't need a massive hard drive really. I'd recommend looking into solid state drives, one for your operating system and programs, and a whole separate one for use as a scratch/temp disk on Adobe and Flexi. The primary storage subsystem - traditionally mechanical hard drives - is actually the slowest part of your computer system when it comes to throughput and accessing and saving files. The faster you can make this, the faster the entire computer operates. This is the benefit of SSDs as they are many times faster than your traditional hard drive. Be careful, though, as they come in smaller capacities than standard hard drives. I've seen people making the transition to SSDs for the first time purchase too small of drives and end up having to upgrade again very quickly because they were running out of space. I'd recommend no less than 120 GB SSDs for each the operating system drive and for the scratch disk drive. Preferably a 250 GB SSD or better for each! And not all SSDs are created equal as well, there are some which have had a proven history of issues or performance problems where others have built a reputation of reliability and speed!
