Depends on the video you're editing and the video editing software you're using. Ask someone with experience of using the same package and workload as you on different bits of hardware.
Generally not a lot unless you're doing it to professional standards. Many home video editing packages (like premiere elements) don't use the GPU at all, only to a level that can be handled by everything or only for it's built in encoding hardware (of which Intel's is one of the quickest anyway).