You cannot SLI a GTX 660 Ti with a GTX 970. Cards in SLI must be the same model.
The GTX 970 is about 60% faster than the GTX 660 Ti and has more VRAM and other new features.
Two GTX 660 Ti cards in SLI would have a peformance boost of about 60% over a single GTX 660 Ti.
This brings them close to the same processing power as the GTX 970, but the effective 2GB of video memory will be an issue at higher detail settings.
There are also compatibility issues with SLI in many games.
The GTX 970 as a single card is a much better upgrade than a second GTX 660 Ti.