if your budget is 600 and not 1600 then a gtx 680 or amd 7970 would be good. and if your budget is 1600 then you might be able to squeeze 2 690s in. and if your budget is 160 which is the most likely price range then a amd 7850 would be the best
if that $1600 is for just the graphics, you could get the gtx 690, simply put, it is the fastest single card on the planet. if not, i'd recommend 2 gtx 660s in sli, which cost about $460, but offers performance better than any single gpu card (not counting gtx 690). if you want AMD, which I dont recommend, the 7870 is amazing performance per buck.