Video Editing Computer Home Built

For $1k you can get an i7 920, a good 6-12GB of RAM, and a cheap graphics card. You won't be needing a good GPU since most video editing programs are CPU oriented.

Questions:

1. Do you plan on overclocking (I would!)
2. How much space do you really need?
3. Do you already have one or two good, high resolution monitors?

+1. If you're not a gamer, a pretty minimal GPU would be fine. Even a 4650 may be more than you need.
If this is a for-profit activity, you probably want enough hard drives to use RAID-1 or RAID-5 for some protection from hard drive failure, AND another drive for backup.
An Intel 920 pretty well bastes the AMD PII 940 at video tasks: http://www.anandtech.com/bench/def [...] =47&p2=80.
Choose a solid PSU from Antec, Corsair, PC Power & Cooling, Seasonic, or Enermax. I put in a sample i7/920 configuration including 5 SATA hard drives and 2 optical drives, a TV tuner card and a couple of other devices, and got 411W at the PSU calculator at http://www.extreme.outervision.com/PSUEngine. In that scenario, a good 500W PSU would operate efficiently and allow for expansion.