No choice here, you'll have to do an nvidia board for sli. the 790gx will only support crossfire x, which is 2 ati cards. The two technologies are competitors, and are not compatible with one another.
The only boards I know of that can do crossfire and/or sli would be the intel x58 series motherboards.
ASUS has a good reputation, except for if anything goes wrong, they have absolutely horrible customer service. But go ahead and get an nvidia based sli board.