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Snostrom8 From my experience a single 980 Ti is a great GPU for your goal of 1080p @ 144 Hz, therefore I presume a GTX 1070 is an equally good graphics card for your goal. But the GTX 1070 is even better: One generation newer architecture, and lower power usage. My estimation is that you'll receive approximately double the FPS with the 1070 than with your current 970. I'm stating an estimate because while I previously owned a GTX 970 (MSI GTX 970 Gaming 4G), I've yet to get my hands on the GTX 1070. Off subject, I'm still really annoyed about Nvidia's mishandling of the pricing for the GTX 1070/1080 GPUs. Today (June 29th) happens to be the release date for the AMD RX 480. While I don't think that's an appropriate substitution of a GTX 1070, my hope is that it'll put downward pricing pressure on 1070. We shall see very soon if my hope turns out to be true.
Btw: You mentioned high/medium settings in Witcher 3. Were you aware that there is a high graphics settings in that title called "Uber" ? Google it. This setting is hidden and not accessible from the game menu but from the INI file. There is also a free utility that can make the minor change for you, in order to enable it. My guess is that CD Projekt Red didn't want reviewers and hard core gamers enabling this option upon the title release and thereby lowering the FPS.
One other thing: As far as your question about the i5-3570k bottlenecking your potential GTX 1070 in Witcher 3, I honestly do not know if that'll happen. It's certainly possible. For example, even a current generation i5-6600k will bottleneck Fallout 4 with a GTX 970, but that example is an outlier. Were it me in your situation, I'd keep my eye out for an EVGA GTX 1070 SC (or FTW if it releases at a reasonable price) and then in the near future I would consider selling the four year old i5-3570K and GTX 970 used on Ebay. The i5 and GTX 970 can fetch approximately USD$125 and $200 respectively. With that money you could purchase a i7-6700K. If you're a professional (not just competitive) gamer, then you'll want as high a GPU clock rate as possible. But if you're simply a competitive gamer, and/or twitch streamer then I would suggest that you go for a 6 core Intel i7 instead. I currently own the Haswell-E i7-5820K and I love it. The latest replacement is the Broadwell-E i7-6800K. An acquaintance of mine, Joker (of Joker Productions), recently stated in a podcast that the 6800K doesn't overclock as easily as the 5820K. In this example I don't believe he was able to achieve the 4.6 Ghz he witnessed on the 5820K, and it took more voltage on the 6800K to get to a lower clock. I know that's not definitive, but it's just something to consider. Both chips use the same LGA 2011v3 socket, while the Skylake 6700K uses the LGA 1151 socket.
Ebay query: i5-3570k (used/sold)
http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&LH_ItemCondition=3000&_nkw=i5-3570K&LH_Complete=1&LH_Sold=1&rt=nc&_trksid=p2045573.m1684
Ebay query: GTX 970 (used/sold)
* You didn't state that brand of 970, so the results lack pricing specificity.
http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&LH_ItemCondition=3000&_nkw=gtx%20970&LH_Complete=1&LH_Sold=1&rt=nc&_trksid=p2045573.m1684
Intel i5-3570K Ark webpage:
http://ark.intel.com/products/65520/Intel-Core-i5-3570K-Processor-6M-Cache-up-to-3_80-GHz
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