If it's for video editing a faster videocard won't help much, unless you have video-editing software that can make use of Opencl or Nvidia's CUDA. In general for video editing you need a faster processor.
Anyway if your PC is the one HammerOn mentioned, then you can only use one videocard, just because there's only one PCI-E x16 slot on the motherboard. While the Core i7 860 with 8 threads should be enough for fast video-editing, but you could use a boost from a decent overclock, but I guess the motherboard in that HP PC wouldn't allow it.
Anyway, if besides video-editing you also want to play demanding video games, then a GTX 670 and a Corsair TX650 V2 PSU would be more than enough (depending on the screen resolution of your LCD monitor or single/multi-monitor setup).