For everyday computer use, processing power is rarely a consideration anymore, but when you're looking at a workstation system for more advanced technical tasks, all those specifications become vital. These are the features you should look for.
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Your exact needs will obviously vary depending on the use the workstation will be put to. That said, the space has become highly commoditised, and the specs suggested below cover a wide range of possible use cases. The difference between a workstation, a high-end gaming PC and a server isn't as clear as it used to be -- all will use many of the same components -- but major manufacturers still generally draw the distinction and list workstations separately.
While the workstation market was once divided on RISC versus CISC lines, that's no longer a relevant distinction: we now live in a purely x86 workstation world. In that space, your choice is between Intel's Xeon and AMD's Opteron, with Intel options somewhat more dominant.
Current Intel models are likely to run the Xeon E3 (either 3.10GHz or 3.40GHz; go for the latter if you can afford it). The cheapest models will be dual-core, but we'd suggest going for quad-core as a minimum to ensure you get a decent life from your investment. For heavy workloads, multiple sockets can help you beef up available power.
While you'll see workstations for sale with 8GB of memory or less, that doesn't make sense given the workloads involved. 16GB is the absolute sensible minimum, and it's not unheard of to go up to 128GB for more demanding applications. Make sure your workstation supports ECC (error correcting code) to minimise the risk of errors.
You're unlikely to encounter a workstation with a conventional hard drive smaller than 500GB; go for 1TB or more if you can afford it. Workstation operators typically work with large files, so local storage is beneficial. Look for RAID support to automate failover in the event of a drive problem.
Using an SSD will give you much better performance, but you won't have such large storage volumes. Stripping multiple 256GB SSDs can be worthwhile in some scenarios.
What you choose here will depend on the budget, application and the number of existing screens you have sitting around that can be reused. Workstation users often require a multi-monitor environment, so make sure the graphics card you choose can support that and has appropriate outputs. Nvidia's dominant model in this space is the Quadro; AMD's FirePro is an alternative.
While it is possible to recycle existing displays, workstation users will typically spend many hours in front of their machine, so don't skimp here by forcing them to use an old, low-resolution display. Bundled monitors with workstations are also often cheaper than if you pick them up separately.
Most workstations will ship with a 64-bit version of Windows 7. You'll often have the right to upgrade for free to Windows 8; whether you want to do that is a matter of personal choice. If your enterprise licensing already covers your OS or you're planning to run Linux or Unix, you can save money by ordering a hardware-only workstation, though this may require a little cajoling of the vendor.