I have 2 x 780 gtx in SLI and its still a flying machine at standard clock.
Power is more but thats the only downfall. Heat isnt an issue with anything past the 680's.
I run a large monitor at 2560 x 1440 and EVERY game I own fly's on max settings (and I do mean max).
However, as I am a tech freak, I will be getting 2 x 980's shortly.
The above poster talks about "SLI having problems of its own". I am interested to hear about this ? I have ran SLI in the past 3 computers that I have built, buy pretty much any worthwhile title on Steam and elsewhere and have never had ANY problems pertaining to SLI (or graphics generally).
SLI isn't like it was in the old voodoo days, its nothing to be afraid of and works out of the box with NO issues....
For me, its a NO brainer...get the 2 x 780's - future proof for ages and faster than a single 980 by a long way...
You dont need the 780 ti in SLI either, its overkill, so your upgrade can be done quite cheaply now.