I was running a GTX660 that was struggling a bit.
Couldn't really justify the cost of a 900 series card so I got another GTX660 fairly cheap and went SLI.
This config is working fine with 64 bit Horizons, driving a 2560x1440 monitor at good frame rates, with most graphics options at Ultra or High.
Depends. Switching from Titan original/780 Ti to Titan X/980Ti doubles the frame rate in ED (100% increase), while power consumption is about the same, whereas Titan/780Ti SLI increases the frame rate by ~70%, at the same time power consumption is doubled.
I am not sure whether this would apply to 770 upgrade to 970, however, it is likely that the latter is going to provide the same performance as 770 in SLI. However, 970 is going to use way less power, and it does not require a very powerful PSU.
However, I'd say that buying Maxwell GPU now would make sense only if the currently used GPU fails. Otherwise, it should be possible to wait a little longer until the reveal of the new GPUs.