From what I've read, the main benefits of the 1080 can only be seen with 1440p/4K gaming, so if you're already getting great performance with your SLI setup then you may be okay for a while. I mean are you going to see a world of difference with 140fps versus 120 (asks the guy who has no experience with either)? I mean yes at the end of the day the 10x0 cards are very powerful and great bang for the buck but you're pretty set for a while. A single 970 is pretty damn powerful and SLI support is common on most major releases. I just think given the hassle of getting rid of the 2 cards then trying to sell them (Which are going to have lower return rates once the 10x0 cards are actually released) and then having to probably spend more money on the card isn't worth it at the end of the day. But again, it's your money, at least it is until I get that algorithm working that can tap into bank account numbers via Xbox Live Gamertags...
Yeah, I'm not planning on doing anything right away. I don't want one of the reference models straight from Nvidia and that is all that is guaranteed to be available at launch. If I bought a new card I would definitely be waiting until the partner manufacturing companies like EVGA start putting out their own models with factory overclocks and custom cooling systems. These cards sold buy EVGA, ASUS, MSI, etc always end up being faster, quieter, and cooler than the reference models Nvidia and AMD design, typically without costing any more.
If these third party options aren't available from day one it will probably only take a couple weeks before we start seeing them. It's possible that the resale value of my 970s will dip quite a bit after the new cards release. I've already talked to a guy that owns a local computer store after seeing that he was looking for used 970s and 980s on Craigslist. When I talked to him on the phone he offered to buy one of my 970s for $225 or the pair of them for $400 cash.
If I can get $400 cash for my 970s when I paid $330 for one of them new about year and a half ago and got the other one free I would probably do it. You are right that SLI support is pretty good these days but there are still some games that don't support it, like everything in the Windows 10 Store. If the reviews end up confirming that a single 1070 is slightly faster than 2 970s and has twice as much VRAM I will make the swap as long as his offer still stands. The reason I didn't do it on the spot when he made the offer is that there were no real reviews yet and I didn't want to make my gaming PC completely useless for a month while I waited for 1070 availability.