i'm not sure what you mean by dual channel gfx. do you mean a graphics card with dual output? (output to 2 different monitors)
or do you mean SLI / Crossfire? combining 2 different video cards for the sake of power. for sli/crossfire you need a special motherboard (either an SLI or crossfire motherboard). if you want to spend $600-700 on videocards, i suggest you don't do SLI/crossfire. get one really good video card (7900gtx or x1900xt).
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius