Hi Robert and others finding this thread,
I'm composing a build for a workstation to support consulting. At work for the past 12 years I've had daily use of various Xeon flavors of engineering workstations. The "corporate" standard build for CAD designers is typically what I draw from---and sometimes help IT staff tune the specs for.
The newer ArcGIS Server (10.1 and up) seem to run much faster for map rendering, but use much more memory; where 2 GB/thread used to be fine, now that need is edging up toward 3.5GB/thread.
On workstations, I've been rather pleased with SSD system drives, although I'd say that after a few months their difference versus a 10k-spin hard disk is not overwhelming. I'd spend the extra $ (or local specie) for an SSD system drive and make sure to have plenty of USB 3.0, or their successors for fast external commodity disks. Maybe don't even have a spinning hard disk inside the case. If you are doing a lot of raster work, it might be worth considering the throughput advantage of a pair of 10k spin drives for separate read and write bandwidth.
For reliability, consider assembling your own external drive of Network Storage class (like Western Digital Red), and only use the Costco-type drives (usually WD Green or frail Seagate models) for backup and exchange.
For ArcGIS Desktop use, sure we're limited to a thread or two per process. But if you have a big load of processing to do, consider this trick to use up to 15 threads: copy separate File Geodatabases for each process, and then launch as many ArcCatalog instances as you need. If each one is working on a separate tile, you can load up a 16-thread workstation to whatever level is tolerable. Before long, you'll need that pair of 10k drives to handle the storage bottleneck.
So in general the clock speed and generous memory is the first best choice, but if your workflows involve bulky geoprocessing, be that raster, or map cache rendering, hydrologic modeling or contouring, it can be very reasonable to consider the advantages of a 12-thread workstation over an 8-thread.
The Xeon chips in a unitary processor system don't seem to have performance advantage for ArcGIS; they don't overclock but should be very reliable at whatever speed they're rated for. For a GIS workstation these days it seems unnecessary to have multiple CPU sockets. Overclock an i7-3930K and it might be more stable and longer-lasting than an i7-4930K; overclocked with extra cooling, either would likely outperform the Xeon E5-1650v2 found at comparable price point. Those LGA 2011-board chips are options for 12 threads. At 8 threads you can use LGA 1150-board chips like the newer i7-4770K, have a bit less L3 cache memory on the CPU itself, and burn about 2/3 the energy (84W vs. 130W design power).
The LGA 2011 chips can handle quad-channel DDR3 memory, so for those it might be worth getting 4x8GB or 32GB of DDR3-1866 (or even faster memory if your ambitions run toward overclocking). For a 12-thread system right now, 16GB is probably enough. If your build is reliable it might last long enough to want more memory in the next three or four years. Making use of quad-channel options you'd be looking at replacing all four banks of memory for an upgrade; consider just getting it now.
The capabilities of graphics cards have become so far beyond the needs of most GIS apps that I'd consider any current or potential gaming desires to inform that decision. Right now, ArcGIS is not tapping into the floating-point power of graphics cards; geophysical software probably is.
Just a single 27-inch to 30-inch display (at 2560x1600) might have plenty of resolution for interactive GIS work, and lessen the need for graphics cards that support two monitors.
The opinions above are entirely my own and do not represent those of my employer. (though it would be great if they did ;^)
-=Brian Q