The key trade-offs for configuring your new video editing computer are the processor (heavy lifting) , the graphics card or GPU (speeding effects), RAM (working directly on sequences), and hard disk (fast access to all the media files). The aim is to provide the right combination of hardware to assist your editing software in providing the best editing experience - with real-time playback of layered timelines, instant preview of edits and effects, and background rendering for export while you continue to work.
Your first priority with today's 64-bit software is to bulk up on the RAM to provide more elbow room to directly process sequences. This is particularly useful if you are working with many layers, tweaking a sophisticated effect like stabilization on a short clip, or moving beyond HD to work with film resolutions. More memory also allows you to keep all your applications open and available as you work, to move seamlessly between editing, effects, and rendering as with Adobe Dynamic Link, then bounce over to Photoshop to update a complex layered raw image, or just to check your email.
But if you typically edit native compressed formats like AVCHD (Advanced Video Coding High Definition), or encode your productions to multiple compressed formats, then a faster processor with more cores will help across a wide variety of tasks. This can not only speed up the processing across multiple formats, for example, but also allow you to continue to edit while your files are being compressed and exported in the background.
Or if you tend to have effects-heavy timelines with multiple tracks with multiple effects, then look to a more advanced graphics accelerator for real-time previews. The GPU also can help with rendering common formats like AVC (Advanced Video Coding), depending on the available support in your software. Or your favorite third-party effects collections may not be accelerated in this way, and instead would lean more heavily on the processor performance.
So can you be a bit more specific as what kind of editing do require?