Don't know if this is helpful but I feel compelled to chime in since I'm also currently upgrading.
I'm coming from 2x 570gtx's and from all the reviews/benchmarks I've seen I was considering moving to a single 780gtx.
I can play BF3 with HBAO and everything on Ultra but with deferred AA set to OFF. I also run Post Processing OFF cause I find it annoying.
My current FPS in game are anywhere from the mid 30s to just under and sometimes touching 60fps. Generally they're in the mid 40s on the 570 sli.
I felt like moving to the 780gtx was kind of a lateral move and not going to really boost things much but it would get rid of the dual card game dependencies and reduce power usage by half or more at idle.
It turns out my brother just got a 780gtx and we've been comparing framerates between games as of late and it turns out its way more than a lateral move as I originally thought.
Here are some numbers comparing these two setups. Bare in mind I'm running a core i7 @4.4Ghz with the 570sli 1.25GB, he is running a 965 i7 @3.2ghz with the 780gtx 3GB.
I am pretty shocked at the differences.
As mentioned above my settings in BF3. 780gtx is running everything Ultra with 2xAA deferred. Resolutions are ~5900x1200 so around 7.1MP per frame.
I'm averaging ~35fps, he's averaging ~50fps (40fps min) with 2xAA and much weaker CPU clock. Of course we know at these resolutions that CPU clock means next to nothing because we're video bound however you'd think it would help feed the card/s a bit.
In the new Hitman, I have everything on HIGH, MSAA OFF, bloom on Normal, Tesselation ON, with 8xAF and average 19.999 fps with dips in the single digits when loading areas.
He is averaging 44fps and never drops below 40 with everything Maxed out and 2xMSAA 16xAF.
In CS:GO which is admittedly "easy to run" I'm averaging 120fps and he's averaging 170fps with everything on max, reflect all, and AA 4x on both setups.
In all cases we're talking about a 25-50% faster aNd everything is running on ultra on his setup smooth and rock steady.
Not so much on my aging setup.
BF3 VRAM usage according to GPU-Z with the above settings on both setups is maxed out on the 570 sli setup on both cards and on the 780gtx with 3GB memory it sits around 2.250GB of VRAM (out of 3).
I guess I'm trying to say two things here.
One, if you're absolutely sold on the 760sli setup. Make sure you buy the ones with 4GB ram or in a couple years you'll wish you did.
Two, if I were you, I'd just get a single 780gtx card, fore-go any sli compatability issues, use less power and have plenty of memory and plenty of horse power.
As far as the dual fan or single fan, if you're not running a water cooled CPU, why would you want to dump all that hot air in your case? Stock HSF, single fan, exhaust it out the back away from your CPU, RAM, voltage regulators,etc.
Pardon the "wall of text"
Either way you'll absolutely love the result.