Perhaps working backwards might help - research the software first. See what are the best set ups per your choice of software.
Some software can take advantage of the GPU (video) and some do not. Those that do, consider if it is the NVidia or ATI (open GL vs Open CL etc.). As well some do better with more powerful CPU. Nearly all take advantage of larger amounts of RAM so which option of hardware can handle more RAM etc.
Since video editing takes up a fair amount of drive space - what are your drive options as far as volume and speed (explore SSD, large drives, external drives etc.)
Last - depending on what you are doing, you may want to ensure consistent colour balance. This requires some 3rd party tool to create profiles (software calibration) such as Color Munki and Spyder just to name a couple as there are a few out there.