Overkill is not a word when it comes to PC gaming

. More power the better.
You can even sell the GTX 950 later on, and put some of the money towards a better GPU if you just want a single GPU solution. The GTX 970 has enormous headroom for overclocking, and with that, I think you will be able to push it further than two 950s in SLI. You'll have to see what works for you, though. I think you should be able to max out your games at 1080p with two 950s, without any problem (the performance with the 950 also seems pretty stable with SLI). It's only with higher resolutions and DSR where you'll need to step up to a GPU with more VRAM - the GTX 970 has 4 GB whereas the 950 only has 2 GB. And, there aren't a lot of Direct X 12 games on Windows 10 that can make use of VRAM stacking as yet.
On another note, I was checking up the power requirements on your 950 a few days ago. Apparently, two of those little cards in SLI require less wattage than a single 970, let alone a 980. That's a good thing if you want to SLI two 950s.