mhmm yeah, 7680*1440 isn't standard size, mainly because the monitors are to expensive and cuz the resolution IS really big!
You need 4GB model for sure! Or you're gonna hit Vram Limit around 4x AA depending on game. And you don't want your Vram to limit your system in 2 Years just because you saved 70 bucks per card!
IMHO its a waste to use two such cards on a PCI-E 2.0 boards on 8 Lanes! Sure it would work! And there are a lot people who will tell you "Its only 2-5% Performance drop! Dont worry about it". Well i would worry about it!
Check out that
test for HD7970 8x PCI-E 2.0 = 4GB/s = 4x PCi-E 3.0 !
I'd go for a LGA2011. 3930k or 3820. 2x 16xPCI-E 3.0 Lanes ! No problems! For TriSLI you'de get16x/8x/8x but as you see in the chart, 8x PCI-E 3.0 is *nearly* no loss!
You might want to Try 2 GPU's at first, and then buy a third if you're unhappy with the FPS.
P.S if you wanna try out how much Performance you will get on 7680*1440 resolution you could try
that tool