Hi,
Sorry haven't read the whole thread, (trying to talk to the missus and littleun at the same time

) and understand you are thinking of a 780.
I've had first hand experience of running 760, 760 sli and 780 sli on Nvidia Surround, so I can give you my experience.
Two 760's in SLI will run a Nvidia surround set up on 5760x1080 pretty well indeed, (or 5910x1080 with bezel comp).
As an idea, 760's SLI'd will run most of the latest games with good settings. Games like Mass Effect 3 and Borderlands 2 can be run with everything on full in surround with just ONE 760. When you start looking at Crysis 3, Black Flag, Arma III etc though you wouldn't be able to run any of them decently with one.
With two 760's I could run black flag at max res, with most detail ramped up, advanced physics though caused frame rate issues, and V sync.
Crysis 3 wouldn't run at max res and with AA on full etc, had to compromise on rez with that one.
Other arcadey games such as batman series ran really well. With 760 sli as long as you compromise with v-sync, some details and filtering you can run pretty much everything.
One 780 can run surround ok, not quite as good as 760 SLI, but pretty reasonable, you still won't max out crysis or Arma etc though.
Two 780's just about manage crisis with everything on full, but even then the VRAM is totally maxed.
Depending on what you want, if you can SLI with another 760 you could be pretty happy with that set up, should be perhaps 20% better than a single 780 (benches I was looking at showed two decent clocked 760's performing on par with a titan).
If you want to know anything specifically about running the surround set up let me know.
Cheers!