are you doing color correction in premiere, speedgrade or other? speedgrade and others will use the gpu more heavily... if you are doing 4k, the gtx 970 is a good idea. depending how much footage you have, replacing the the 1tb hdd with an ssd sounds better than a new ssd for scratch/cache only. you could use the 1tb for backups...
if you are running windows, you can use the resource monitor and gpu-z to help determine which to upgrade first. GPU-Z Downloads