High-end graphics cards: is it needed or useless for video editing at i5 machine with quicksync?
First question to ask is:


Does your video editing software actually use your GPU at all in the editing process? If it isn't written to use CUDA then you will not benefit from getting a better video card, other than being able to view it in higher quality on the screen.


For software that can, all of those factors are important:


Memory size is likely to be the most important as working with all relevant data in memory is faster than having to import from disk.
Memory channel size is also important, as is the speed, for ensuring you minimise latency when reading video data.
CUDA cores are probably the least relevant when editing, as this is not a hugely intensive task. They would be much more important when rendering, or carrying out complex image manipulation.


update based on comments

Adobe Premiere Pro does use CUDA, through the Mercury Playback Engine. From this post on the Adobe forums, whether a frame can be processed by CUDA depends on the size of the frame and the amount of RAM on the graphics card (VRAM), which supports my point that the most important is RAM size.