Hello
According to Nvidia the minimum size power supply to run a GTX 970 should be a 500W. But a little headroom never hurts and investing in a good power supply can actually save you down the road, since you can carry it over from build to build. A cheap power supply might also damage components in case of a power surge or something.
So I would recommend that you invest in a quality power supply of about 700 - 800 W range so you have a little headroom as well. You never want a power supply to be running at it's max constantly.
Hope this helps