I mean, it should be fine. It will run a little warmer but you should be just fine with it. Your idle temp won't change much...1-3C or so. You loaded temps could increase by about 10C which does approach an uncomfortable zone(80C+) for these CPUs. No way to know until you try it. I personally think it will do just fine. I also personally think you are not going to notice any game changing differences in between these two CPUs. It will do a few CPU intensive things just a little bit faster. In games you won't see much of a difference in FPS.
You could OC your 920 to 3.4GHz very easily to see what your temperature profile is. The 970 might be a just a little better than that but it might just be identical to a 920 @ 3.4GHz. Pretty much every i7-920 can hit 3.5GHz no matter what silicon you ended up with.