iXBT Labs posted a guide about choosing an office PC:

General recommendations on how to choose an office PC configuration can be given only with reservations to particular circumstances: sometimes it's the budget and the number of workplaces, and sometimes it's basic requirements to performance. If we proceed from minimal requirements to responsiveness of the operating system and minimum office software (text editor with a spellchecker, email client, web browser, and necessary security tools (antivirus/antispy, firewall), even processors from the start of the list will cope with the task: dual-core Semprons and Celerons. And processors that score more than 100 points in SYSMark will be subjectively fast in such tasks. In fact, further improvements in OS responsiveness (the time it takes to start programs, to navigate folders, and search for files) depend more on HDD performance than on CPU speed. If you work with graphics data, long Word documents, archived data, the optimal choice will be configurations that score 135-145 points. And finally, those users who deal with 3D modeling(even such light tasks as faced by a manager in a furniture store) or video editing should get a processor that can score 145 points or higher. A processor with 3-4 cores will be preferable.

What concerns the other components, even an entry-level computer needs 1 GB of memory. But Windows Vista needs at least 2 GB, which is also the optimal memory size for practically all office PCs. Except for workstations that deal with professional 3D rendering or CAD modeling (which are not considered office computers, although they belong to this group). A graphics card is the first candidate to save money on. There is even no need to install it sometimes, just choose a motherboard with an integrated graphics system. In case of such chipsets as AMD 780V/780G, you will get good basic functionality in tasks that require 3D hardware acceleration.

And finally, if you buy a discrete graphics card and a motherboard without integrated graphics, there is no reason to choose a motherboard with an expensive chipset and several graphics ports, for example, motherboards with AMD 790FX. The top chipset for office motherboards is AMD 770 or Intel P45, even if you install a powerful processor with three or four cores. It may seem as plain as daylight, but we still come across office configurations based on High-End chipsets. When you have a strict budget per office computer, such a configuration will apparently leave less money for a processor. So this computer will demonstrate low results in tests, and it will be really weaker than a computer with a powerful CPU and an inexpensive motherboard. Such chipsets as 790FX or Intel X48 should be used in computers for games and specific professional applications that can use several graphics cards or other high-speed devices installed onto PCI Express x16 slots.