Before you change CPU, you shut try overclock that I7 920. They tent to be solid overclocks if cooled properly. I used to have a I7 920 before i swapped it out with an I7 980X i got dead cheap not so long a go. My I7 920 where good for a oc op to 4.4 GHz and a dayli usable overclock of 4.2 GHz with in Intels safe Vcore range with an good aircooler and run overclocked 4 years of the 8 years i had before i sold it and it is still alive and kicking today. Most I7 920 is good for 3.8-4 GHz and some are also like mine good for even more.
Before you change CPU, you shut try overclock that I7 920. They tent to be solid overclocks if cooled properly. I used to have a I7 920 before i swapped it out with an I7 980X i got dead cheap not so long a go. My I7 920 where good for a oc op to 4.4 GHz and a dayli usable overclock of 4.2 GHz with in Intels safe Vcore range with an good aircooler and run overclocked 4 years of the 8 years i had before i sold it and it is still alive and kicking today. Most I7 920 is good for 3.8-4 GHz and some are also like mine good for even more.
Well its up to you, but it would save you some money. But if overclock is not your thing when change CPU cause it will be a bottleneck if run stock with a GTX 1070.
If you upgrade CPU why not get the latest out there. Like I7 7600K or I7 7700K. AMD up coming RYZEN is maybe also some thing that would be worfh waiting for.