I currently have one gtx 780, and have found a used 780 for $250.
I also found a used 970 for $280. Which would be better, 780 sli or 970 sli?
I currently play games at 1440p, with plans to move to 4k.
Two 970's would be better, but you still would have to get another 970 for sli . your looking at 250 now to sli your 780 or 600 or more to go with the 970's in sli. I would get the 780, but if you have the cash, get the 970's. Sell your 780.
I bought a 980 to drive off an UHD TV after it's launch (which replaced 680's) and it is a great card, however, I wouldn't suggest it simply due to it's price. As for the 780, I wouldn't get near it simply due to it's inefficiency in GPU power compared to 900 series, and inefficiency in wattage levels as well.
Go with the 970's. It's not like you're going to hit that kind of vRAM usage most the time, even at 4K. Even if you do, it would most likely be at pretty high settings which you could reduce. Hell, personally, I don't even game nowadays... not to mention it's a problem that seems like it could be fixed.