it is a single card. in so far as its a card, which is single.
regardless, a 750w psu is not an extravagant expense. its what i would consider a sweet spot to be honest.
+1, l put a HX850 in my build 4 years ago, no matter what l put in at that time with in it will be ok, sli. xfire, water cooling and have no worries.
Your PSU is your No1 component for running x amount of components, stabile voltage's, overclocking, my next build will be at least a 1000w.