If you're going to change your motherboard and case with the GTX 1070, I would say to you start your upgrade with the MOBO and CPU, but still I believe you would still run the actual games with the 1070(with bottleneck), You may receive 1%-10% of bottleneck on your GPU-CPU, your CPU is a bit of old but the requirements for a GTX 1070 is a i7 3.2Ghz CPU, it would be a go for it. I think you should go for it and wait for CPU so you can see the results.
OBS: I own a GTX 1080 with a i5 2300 2.8Ghz, I have bottleneck of a 30% I would say. It works perfectly, this month i will upgrade my PC with the new i7 6700k, ASUS B150-Plus and a lot more...
I took the same path as you have right now I'm really happy about the GTX 1080 is a great graphic card, my friend own a 1070 and got a i5 2320 and will upgrade it in the future.
Hope I was helpful.