Good rule of thumb is always buy the faster single GPU and add another later. What resolution are you running these games at? Both the 760 and 7970ghz ed. should be able to max those games out on a single card at 1080p resolutions.
The 780ti will give you a much smoother experience than the 7970ghz ed. Xfire and use less power doing it/create less noise.
From a performance point of view,the twin 760's win the battle.However,they will generate more heat,noise,draw more power,you can get stuttering in games.If you don't plan on going higher that 1080p,a single 760/7970Ghz would be enough,as dwatterworth suggested.
AMD cards generally draw more power than nVidia cards,so yeah the 7970Ghz draws a lot of power.
You'd be better off getting the stronger 780ti.nVidia has better driver support(driver for xfire are a mess),and you won't experience stuttering with a single GPU.
And because it's a single GPU,it will run cooler,quieter.Not to mention the awesome power of a single GTX780ti.