is Dual GTX 670s good for video editing? or should I go for one GTX 690... and should I get cards that have 4GB of VRAM or is it ok to get cards that only have 2?
A single 670 gtx will be the best for video editing, usually nvidia with cuda cores, are better for that department, a single 670 2GB gtx will be suffice either for gaming at 1080p or editing.