no question. nvidia. as an fyi:
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
i personally run dual 590s, with a i7 2600k. the only issue if you plan on running quad SLI, is getting a mobo that has adaquete space for cooling. given the heat of a I7, with the 590's the only solution for me was to watercool.
if you want the best, EVGA 590's. and watercool it. you will not regret it.