I currently have a rig that is about 5 years old running the GTX 680.  I am looking to upgrade my gaming to 1440p after being on 1080p for quite some time.

Would a GTX 1080 be a good upgrade or would waiting for the 1080ti version be better or would the 1070 be sufficient?

I am also a bit worried about possible compatibility issues. Perhaps a full rig update is better? Although the performance upgrade from 2700k to a 6700k does not seem to really justify the cost.
 
Current Rig Status:


OSWin 7 Home 64 Bit   MotherboardGigabyte Z68MX-UD2H-B3  ProcessorIntel Core i7 - 2700k LGA 1155  CoolerHyper 212+  Hard DriveCrucial M4 256GB SSD  RAMCorsair Vegence DDR3 1600 4x4GB  Graphics CardEVGA 680 GTX  Power SupplyPC Power and Cooling Silencer 910W    CaseCooler Master HAF 32  Monitor Dell Ultrasharp 2412M 

Thanks in advance!

I currently run two 680s in SLI on my 1440p monitor. My games run fine. Some may require tweaking the game settings to get optimal frame rate. But they still get the job done. Going from 1080 resolution to 1440 resolution does make a different on the system. So the more VRAM you have the better the experience. Your system in my opinion is still strong. All you really need is a 1440p monitor of choice GSYNC helps alot in this case. And at least a 970 or above.
 
2x680s benchmarks are pretty close to a single 970.
 
http://www.3dmark.com/fs/6933744 Overclocked cards and CPU.
 
You can completely over haul your system or you can upgrade the video card and get a nice monitor and your good to go for a little while longer.
I am ready to jump on 1080. I am just not really all that excited by founders edition as I am sure the better versions will drop in the not to distant future.
But in all honestly my system still kicks plenty of arse in all the games I play.
 
My kids purchased the "Founders Edition" as a Fathers Day gift for me. 
 
 
Rainbow 6 Siege
BF4
Overwatch
Black Ops 2
Hearthstone
to name a few....
 
Good Luck