gtx 970 vram problem(so which gpu should i get)

VXGI is actually (believe it or not) cross platform since it uses DirectCompute instead of CUDA, so it is a viable for upcoming games. The performance is the biggest right now, but Mike from Nvidia has tested it on a GTX 670 succesfully with a decent framerate. It will need more refinement but it looks promising.

Nvidia is currently implementing all of the Gameworks library’s into UE4, and have stated they will make them cross platform compatible once the base integrations have been completed. So far WaveWorks and FleX have been integrated, and VXGI is due to be released any day now. For more info on all of see the following thread: NVIDIA GameWorks Integration - Asset Creation - Epic Developer Community Forums