i7Baby :
Will the GTX 970 @1440p perform as good (or better than) the GTX 660 @1080p? - yes - if your CPU and ram support it.
How long do you think it'll take before I need to upgrade again (running AAA titles at medium-low 60fps)? - You'll never NEED to. But you'll want to in about 2 or 3 years.
Thanks so very much for the instant answer! My CPU is an i5 3470, and is ranked at Tier 1 in the CPU chart, so CPU-wise I'm good. I have 8GB of DDR3 1600MHz ram, so RAM's fine too.
As for performance in games, do you mean to say that in three years time AAA titles will run at the settings I said, at 1440p? If that's the case, I might as well just wait and see what the 390X has to offer...