nV cards are less reliant on CPU grunt to extract peak performance from them, especially in DX11 games. AMD cards require more CPU power to perform well. Going forward with DX12/Vulkan games this will change and is already seen in the few DX12 games available where AMD cards gain lots of performance in most cases while nV cards remain equal or lose performance vs DX11.
Just have a look at that second post I linked in my previous post ("show its age"). An i7 960 (same CPU generation as yours) and an i7 4790, both at 4GHz. The i7 4790 rig is usually 20-30FPS above the results the i7 960 rig is producing. Same videocard on both (R9 390). Doom is a well optimized game that gets >60FPS at 1080p on most hardware. Other not so optimized games will slow down on your rig vs a newer platform.
If you upgrade to a 1070 you'll get better performance out of it over your 970, sure. Problem is, you won't be anywhere near utilizing the 100% of your new video card. You'll have to upgrade your platform to something newer to get that 1070 performing as well as it can. An i7 6700k + fast DDR4 (3000MHz or higher) will get 100% out of your 970 and will probably max out a 1070, too.
At the same MHz a 6700k is on average about near 50% faster than a i7 990x at this point, and it shows. Yes, you get 4 cores vs the 6 you have now, but these are so much stronger it doesn't really matter.
Happened to me a few years ago, I had a shiny new Radeon HD 6850 having upgraded from a 4850 that had died. I was using a Core 2 Duo E7200, overclocked to 3.8GHz, nice system, but a few years old already. I remember not being satisfied with the performance in Assassin's Creed II in the cities, minimum FPS were low and you could notice the game stuttering. Other 2010 era games ran fine but were starting to behave like this.
I thought I should've stretched to the 6950 and lamented buying the 6850, but then I decided to upgrade to a i5 2500k (4 years newer CPU tech, and a quad core vs my E7200 that was a dual core) while keeping the same video card. FPS were rarely below 50-60 now in that game, fluid game play everywhere. Funny thing is, the GPU usage meter in the E7200 system showed a pegged 100%... clearly it's not a reliable indicator when the bottleneck is somewhere else.
So yeah, if I were you I'd keep the 970 for now and first overclock that i7 990x, see if FPS get better in other games than Doom where you might not be satisfied with the performance. If not, time for a platform upgrade.