Before I do initiate my "14 day satisfaction guarantee" or my "no coil whine guarantee", how likely is it that I'd break the 3.5GB limit with SLI 970's at 1080p and max settings over the next 24 months? I'm guessing pretty likely? I know I certainly will with my CUDA rendering which is what I primarily bought the cards for.
It would all depend on how high you set MSAA.
When I built my "4k ready" rig I didn't want to sacrifice hardware so I bought a 1080p 27" monitor for £220 or so fully expecting to give it to my wife and replace it with a 4k or 1440p monitor.
Running a 27" display @ 1080p means I need a lot of msaa to offset the low res to display size ratio so I run 8x MSAA on titles such as Crysis 3.
I know that adding lots of AA to a game really smashes the VRAM.
A couple of years back I had two 670s in SLI and 4gb system ram. I loaded up NFS Shift and maxed out the MSAA and got an Nvidia message in red plastered all over the screen saying "not enough memory to run the chosen setting, settings will be automatically reduced.
So, I would guess it all depends on what sort of aliasing you use really.
IMO the 970 is a 1080p card any way, so it should be fine

but know this; Nvidia always launch their cards with just enough of everything "for now".
They want to keep you coming back.
VRAM is obviously reasonably cheap. On the 4gb 670 for example the premium was only £40 or so. But Nvidia do not want your card to last four years, they want it to last until the next time they have a new card that they want you to buy.
AMD are not as shrewd and you usually get a far higher memory bandwidth and a fair chunk of actual memory.