No one can answer how much space will be sufficient for you. I have 8.5 TB and I am over 90% full while some people with 500gb can't fill up the disk.
Unless you shoot hd video and store lots of uncompressed video then I think 1 TB should last you for a while, you can always add more space if you feel that you are running low.
I don't think 970 will drop in price anytime soon, its already listed very low, the expected MSRP for 970 was $400. I would highly suggest you get 970 as its a superior card from r9 290 in everyway. (More Performance, Less Power consumption, Less heat, DX12 support etc)