Is it worth to upgrade from a gtx msi 560ti oc SLI system to a new gtx 670/680?
Good question, with a simple answer: no. Your GeForce GTX 560 Ti setup is still quite good right now, and would be close to the performance of a GEFORCE GTX 670 or 680 on their own. If you're after more grunt, I'd wait for next-gen GPUs, or look at upgrading your monitor setup.
Say you're running a 1920x1080 resolution, your GTX 560 Ti SLI setup is fine for virtually all games out right now. Your SLI setup would only need to see an upgrade as your resolution goes up. Your VRAM would be used up, and then you'd have a reason to upgrade your GPUs. I would suggest investing the money from a GPU upgrade into two more screens, giving you a Surround Vision setup.
Your GTX 560 Ti SLI setup would still be more than powerful enough to run the three screens, and as time goes by, upgrade to a couple of 670 or 680's when they get cheaper.