I'd say get HDD for sure, 1-2 tb, whatever fits into your budget, this is where you dump all your HD porn.
For c:/ or windows, I'd highly recommend getting SSD, even a shit one is miles ahead of hDD, Samsung evo series is excellent, decently priced and great reviews all around.
Because see SSD 1 tb is goint to be batshit expansive, so for data, you get HDD, 1 tb is dirt cheap.
For Windows, as your c:/ partition get 250gb or 500gb, keep that purely for windows, ms office and you know, drivers and anti-virus, chrome etc, the basic but necessary stuff.
Install your games on HDD etc.
SSD windows is blazing fast and smooth and a world of difference really, youre windows will fly.
So tldr: SSD for Windows, HDD for porn.
SSD would be my next immediate upgrade. After that, im going to sit content for the next 2-3 years. I'm more of a dota-2 and CS:GO kind of guy, im not really into single player games like withcer or fall out 4 or batman etc. So even 970 is an overkill for the Dota and CS:GO, but it keeps me plugged in for next 2-3 years. I'm very satisfied with the quality of graphics it outputs.
Also, for the monitor, 970 will do ultra or near ultra for most games at 1080p, for 4k, you'll need to lower your settings. 970 isnt really for 4k displays, you really need to up the ante to 980 ti etc for that.
And so i'd say stick to 1080p monitor. or if you're getting 4k monitor, you play games at 1080p, porn at 4k. That's workable at 970.
Another important thing - see what's the price of ATi equivilant of 970. Sometimes you can get a better deal with better prices, nvidia charges bit of premium. The reason i didnt get ATi was it's not really that well distributed here and if something goes wrong, i cant be arsed around for support.