Wow, that is one hell of a power supply. I think a 660gtx is a good upgrade. I never recommend SLI, but I assume you are not having issues with it. I would look at some benchmarks and see which cards are fastest in the games you play.
Skip the GTX 660 and buy a better card maybe a 7850 or 7870 if your budget allows it. SLi 265's are all about worthless now. If you must buy Nvidia I would save my money and buy a GTX 670 and skip both the GTX 660 and GTX 660ti.
It should be a bit faster plus you will be avoiding having to disable SLI in some games and have some more of the features seen in the newer cards...
Also that PSU isn't going to be that efficient even with the 275's, way less with any of the keplers, I would change it out too if you do make the switch... At the end of the day its costing you more than it should to run
A 670 may be bottlenecked a bit by a 965, 660 should be fine