High-end graphics cards: is it needed or useless for video editing at i5 machine with quicksync? I am in search of a graphics card that makes video editing fast.

I think to buy an i5 cpu with a motherboard with quicksync (z68?) and an SSD.

Quicksync on cpu is only for transcoding (h.264 and mpeg2 as I understood).

I suppose that I need a discrete graphics cards with cuda to make faster the rest of video editing. 
I don't care about 3D games.

Ofcourse, I will use software that supports the technologies needed.

The differences among the various graphic cards currently are the number of cuda cores (eg 96), the memory size (eg 1GB), the memory-channel size (eg 128bit), the type of RAM (DDR3/DDR5) etc.

Up to what point, adding cuda cores, and more memory to the graphics helps would help my video editing process?


Is it enough to buy a cheap cuda card (under $100)? 
Would buying an expensive card with more cuda cores, extra memory, bigger memory
channel size, offer significant advantag over cheap cards? 
Could an expensive cuda card even offer no advantage at all, if there are
other bottlennecks at the system?
Does a system with i5, z68    motherboard and SSD disk, 8 GB ram, and windows 7 64 bit have any bottleneck for high-end cuda cards?
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.