Support for the 970 and 980 is experimental and only exists in the latest versions of AE CC. It's as unofficial as it gets. Good luck!
AE uses the GPU for very little. The only thing it really uses it for is to accelerate the ray-traced renderer. But that is considered obsolete, so I wouldn't worry about it. (If you do plan to use the ray-traced renderer, that card series will work to accelerate it in CC 2015.)
That card is an excellent choice for working with AE if you plan to use Element 3d. All reports are it's the best bang for your buck.
Thank you Szalam
Thank you Mylenium