Sounds like a similar tough call I had to make when I was debating between getting a i7 5820k or the i7 6700k (I was sporting a first generation i7 920).
I ultimately decided on the 5820k even though it was a year older. Why? At the time they were the same price but the difference was like the original i7, the 5820k was part of a 'enthusiast platform/build'.
This to me felt more like an upgrade because not only do you get a powerful system with more cores but it also sported the new DD4 Quad Channel Ram and more PCI-E lanes.
I'll point out a few things that may help your decision:
-If all your doing is gaming, I would upgrade to the latest processor (most games barely use 4 cores and we have yet to see DX12 games that take advantage of 6 or more cores)
-PCI-E lane bandwidth issues will be negligible between PCI 3.0 x8 and PCI 3.0 x16, even for Crossfire/SLI (unless your planning on adding other PCI-E components like a M.2 SSD) (see below)
-No real benefits to quad channel ram over dual channel ram for gaming
-For multi-monitor resolutions, your usually GPU limited
Looking back, if I knew all I would be doing is gaming I would have chosen the latest architecture (i7 6700k). However, being able to overclock the 5820k to 6700k speeds I can get similar gaming performance with the additional cores handy for what the future throws at us so I guess its not all bad.
The i7 3770k can definitely still hold it's ground and your not that many generations behind. I would say hold off on a new CPU purchase unless you know for sure the upgrade will benefit your CPU intensive games today. I've heard the next generation of processors will have a drastically different architecture and it may worthwhile to wait a little longer.
I hope that helps,
kiax0001
Edit: After writing this post I read the following article
http://www.pcgamer.com/the-best-pc-gaming-cpus-processors/ which claims 10-15 percent more SLI performance using the 6850k vs 7700. If you like SLI and think you will continue to use it in future builds, it sounds like 6850k is the better high end gaming CPU for you.